###Barty Crouch Jr. has more control being Moody than Imperiusing the real one.

First of all, and perhaps most importantly, Barty Crouch Jr. would be able to strategically plan his actions much better when doing them himself than having to puppet Moody through them. There's a definite benefit in being able to take over the role of Moody, and do everything himself. He'd be able to take quick, spur-of-the-moment action if the situation requires it, something that an Imperiused person following orders, who of his own will would certainly not be doing anything to help the Dark Lord, would be unable to do as well.

Imperiused people require a lot of micromanaging, and would need to be told what to do fairly frequently. When Harry uses Imperio on Bogrod to make him take them into the vault, for instance, Harry has to make Bogrod put his hand on the door to open it - something Bogrod in his right mind would surely know. Harry isn't good at casting Imperio, so a skilled Dark wizard could certainly do better, but in all the other situations the Imperius Curse is used, it turns people into pliable meat puppets, not cunning strategists.

“Make him press his hand to the door!’ Griphook urged Harry, who turned his wand again upon Bogrod. The old goblin obeyed, pressing his palm to the wood, and the door of the vault melted away to reveal a cave-like opening crammed from floor to ceiling with golden coins and goblets, silver armour, the skins of strange creatures, some with long spines, others with drooping wings, potions in jewelled flasks, and a skull still wearing a crown.” _{^{- Harry Potter and the Deathly Hallows, Chapter 26 (Gringotts)}}

As Moody, Barty Crouch Jr. had to do several things that required cunning and strategy. Only some of these are: Confund the Goblet of Fire and put Harry's name in it, pretend to console Neville so that he could give Neville a book with information that Harry needed, turn the Triwizard Cup into a Portkey, remove the obstacles in the maze so Harry could get to the Portkey cup easier - and all without raising anyone's suspicion. It would be difficult, if not impossible, to make an Imperiused person do that all flawlessly, without attracting suspicion or notice.

In addition, Moody would likely be able to resist the Imperius Curse after a while.

Relying on the Imperius Curse to keep Moody subdued would be foolish. Even if Barty Crouch Jr. was able to keep him mostly under control, which is doubtful (as another answer by chirlu explains well enough) even him gaining clarity for a small amount of time could be fatal to the Dark Lord's plan, as Moody could use his moment of clarity to do any number of things - write a note and leave it somewhere, tell Dumbledore, hide, and these are only a few possibilities out of many.

Barty Crouch Jr. has more control being Moody than Imperiusing the real one.

First of all, and perhaps most importantly, Barty Crouch Jr. would be able to strategically plan his actions much better when doing them himself than having to puppet Moody through them. There's a definite benefit in being able to take over the role of Moody, and do everything himself. He'd be able to take quick, spur-of-the-moment action if the situation requires it, something that an Imperiused person following orders, who of his own will would certainly not be doing anything to help the Dark Lord, would be unable to do as well.

Imperiused people require a lot of micromanaging, and would need to be told what to do fairly frequently. When Harry uses Imperio on Bogrod to make him take them into the vault, for instance, Harry has to make Bogrod put his hand on the door to open it - something Bogrod in his right mind would surely know. Harry isn't good at casting Imperio, so a skilled Dark wizard could certainly do better, but in all the other situations the Imperius Curse is used, it turns people into pliable meat puppets, not cunning strategists.

“Make him press his hand to the door!’ Griphook urged Harry, who turned his wand again upon Bogrod. The old goblin obeyed, pressing his palm to the wood, and the door of the vault melted away to reveal a cave-like opening crammed from floor to ceiling with golden coins and goblets, silver armour, the skins of strange creatures, some with long spines, others with drooping wings, potions in jewelled flasks, and a skull still wearing a crown.” _{^{- Harry Potter and the Deathly Hallows, Chapter 26 (Gringotts)}}

As Moody, Barty Crouch Jr. had to do several things that required cunning and strategy. Only some of these are: Confund the Goblet of Fire and put Harry's name in it, pretend to console Neville so that he could give Neville a book with information that Harry needed, turn the Triwizard Cup into a Portkey, remove the obstacles in the maze so Harry could get to the Portkey cup easier - and all without raising anyone's suspicion. It would be difficult, if not impossible, to make an Imperiused person do that all flawlessly, without attracting suspicion or notice.

In addition, Moody would likely be able to resist the Imperius Curse after a while.

Relying on the Imperius Curse to keep Moody subdued would be foolish. Even if Barty Crouch Jr. was able to keep him mostly under control, which is doubtful (as another answer by chirlu explains well enough) even him gaining clarity for a small amount of time could be fatal to the Dark Lord's plan, as Moody could use his moment of clarity to do any number of things - write a note and leave it somewhere, tell Dumbledore, hide, and these are only a few possibilities out of many.

###Barty Crouch Jr. has more control being Moody than Imperiusing the real one.

First of all, and perhaps most importantly, Barty Crouch Jr. would be able to strategically plan his actions much better when doing them himself than having to puppet Moody through them. There's a definite benefit in being able to take over the role of Moody, and do everything himself. He'd be able to take quick, spur-of-the-moment action if the situation requires it, something that an Imperiused person following orders, who of his own will would certainly not be doing anything to help the Dark Lord, would be unable to do as well.

Imperiused people require a lot of micromanaging, and would need to be told what to do fairly frequently. When Harry uses Imperio on Bogrod to make him take them into the vault, for instance, Harry has to make Bogrod put his hand on the door to open it - something Bogrod in his right mind would surely know. Harry isn't good at casting Imperio, so a skilled Dark wizard could certainly do better, but in all the other situations the Imperius Curse is used, it turns people into pliable meat puppets, not cunning strategists.

“Make him press his hand to the door!’ Griphook urged Harry, who turned his wand again upon Bogrod. The old goblin obeyed, pressing his palm to the wood, and the door of the vault melted away to reveal a cave-like opening crammed from floor to ceiling with golden coins and goblets, silver armour, the skins of strange creatures, some with long spines, others with drooping wings, potions in jewelled flasks, and a skull still wearing a crown.” _{^{- Harry Potter and the Deathly Hallows, Chapter 26 (Gringotts)}}

As Moody, Barty Crouch Jr. had to do several things that required cunning and strategy. Only some of these are: Confund the Goblet of Fire and put Harry's name in it, pretend to console Neville so that he could give Neville a book with information that Harry needed, turn the Triwizard Cup into a Portkey, remove the obstacles in the maze so Harry could get to the Portkey cup easier - and all without raising anyone's suspicion. It would be difficult, if not impossible, to make an Imperiused person do that all flawlessly, without attracting suspicion or notice.

In addition, Moody would likely be able to resist the Imperius Curse after a while.

Relying on the Imperius Curse to keep Moody subdued would be foolish. Even if Barty Crouch Jr. was able to keep him mostly under control, which is doubtful (as another answer explains well enough) even him gaining clarity for a small amount of time could be fatal to the Dark Lord's plan, as Moody could use his moment of clarity to do any number of things - write a note and leave it somewhere, tell Dumbledore, hide, and these are only a few possibilities out of many.

###Barty Crouch Jr. has more control being Moody than Imperiusing the real one.

First of all, and perhaps most importantly, Barty Crouch Jr. would be able to strategically plan his actions much better when doing them himself than having to puppet Moody through them. There's a definite benefit in being able to take over the role of Moody, and do everything himself. He'd be able to take quick, spur-of-the-moment action if the situation requires it, something that an Imperiused person following orders, who of his own will would certainly not be doing anything to help the Dark Lord, would be unable to do as well.

Imperiused people require a lot of micromanaging, and would need to be told what to do fairly frequently. When Harry uses Imperio on Bogrod to make him take them into the vault, for instance, Harry has to make Bogrod put his hand on the door to open it - something Bogrod in his right mind would surely know. Harry isn't good at casting Imperio, so a skilled Dark wizard could certainly do better, but in all the other situations the Imperius Curse is used, it turns people into pliable meat puppets, not cunning strategists.

“Make him press his hand to the door!’ Griphook urged Harry, who turned his wand again upon Bogrod. The old goblin obeyed, pressing his palm to the wood, and the door of the vault melted away to reveal a cave-like opening crammed from floor to ceiling with golden coins and goblets, silver armour, the skins of strange creatures, some with long spines, others with drooping wings, potions in jewelled flasks, and a skull still wearing a crown.” _{^{- Harry Potter and the Deathly Hallows, Chapter 26 (Gringotts)}}

As Moody, Barty Crouch Jr. had to do several things that required cunning and strategy. Only some of these are: Confund the Goblet of Fire and put Harry's name in it, pretend to console Neville so that he could give Neville a book with information that Harry needed, turn the Triwizard Cup into a Portkey, remove the obstacles in the maze so Harry could get to the Portkey cup easier - and all without raising anyone's suspicion. It would be difficult, if not impossible, to make an Imperiused person do that all flawlessly, without attracting suspicion or notice.

In addition, Moody would likely be able to resist the Imperius Curse after a while.

Relying on the Imperius Curse to keep Moody subdued would be foolish. Even if Barty Crouch Jr. was able to keep him mostly under control, which is doubtful (as another answer by chirlu explains well enough) even him gaining clarity for a small amount of time could be fatal to the Dark Lord's plan, as Moody could use his moment of clarity to do any number of things - write a note and leave it somewhere, tell Dumbledore, hide, and these are only a few possibilities out of many.