Voldemort went through great lengths to retrieve the prophecy that predicts his downfall. Sending Death Eaters, unspeakable, even planting images in Harry's head to lure him in. Why didn't Voldemort just use Polyjuice Potion and retrieve it himself? Surely if Ron, Harry, and Hermione were able to use it to infiltrate the Ministry in DH, he could use it right?
It wasn't just about getting the prophecy, he wanted to fulfil it.
"The one with the power to vanquish the Dark Lord approaches... born to those who have thrice defied him, born as the seventh month dies... and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not... and either must die at the hand of the other for neither can live while the other survives... the one with the power to vanquish the Dark Lord will be born as the seventh month dies...
Yes, he wanted the prophecy. He didn't know if it contained any more information which could help Harry destroy him, or help him destroy Harry. I think he suspected that prophecy held information as to why he couldn't touch Harry, or why their wands connected at the end of Goblet of Fire. He wanted answers, and the prophecy seemed like a good place to start. However, what he really needed was to get rid of Harry Potter.
Now addressing the question, would Polyjuice Potion have helped him invade the ministry of magic? Probably not. Polyjuice Potion takes a month to prepare, and I'm sure that there is other, more versatile magic that he could have relied on if he had wanted to disguise himself. Disguising his appearance would have been the least of his problems when trying to get into the most secret section of the ministry of magic, however. I think most of the time Voldemort spent "trying to get into the ministry of magic," he was learning about what defensive and protective spells guarded it. Certain charms could have easily killed him if he had triggered them. I think the only reason Harry and his friends weren't killed or trapped when they invaded to try to save Sirius, is because Voldemort had gone ahead and wiped most of the traps out, just like in The Philosophers Stone.
I think he also knew that if he did pick up the prophecy, the ministry of magic would have known he is back, because only him and Harry could have picked up the prophecy. Whether the ministry would have kept quiet of not, I think Voldemort preferred to not let them know. The fact that only his closest followers knew he had returned was one of his biggest advantages at the time.
Luring Harry to the ministry of magic would be a win/win situation for him. If Harry fell for it, Voldemort could kill him. If harry came, but escaped, Voldemort would be removed from suspicion if the prophecy "mysteriously" disappeared. If his plan to lure Harry to the prophecy failed, I'm sure he would have taken the prophecy anyway.
Sorry, the first time I read the question I thought you were asking "Why didn't Voldemort turn into Harry with Polyjuice Potion so he could pick up the prophecy.
- Because only the person the prophecy is about can retrieve it.
- Polyjuice Potion only allows you to have the appearance of someone else, not become them.
So, it has been awhile since I have read the books, but I don't remember any deviation regarding the movie where this is concerned. The quote I found is in reference to the films... just in case they are different. I pulled the script from here.
So in the film this is the exchange between Harry and Lucius:
Lucius: "All we want is that prophecy."
Harry: "Why did Voldemort need me to come and get this?"
Lucius: "Prophecies can only be retrieved by those about whom they are made. Which is lucky for you, really..."
And as detailed by Pottermore the Polyjuice Potion only allows you to appear as someone else, not become them:
The Polyjuice Potion, which is a complex and time-consuming concoction, is best left to highly skilled witches and wizards. It enables the consumer to assume the physical appearance of another person, as long as they have first procured part of that individual’s body to add to the brew (this may be anything – toenail clippings, dandruff or worse – but it is most usual to use hair). The idea that a witch or wizard might make evil use of parts of the body is an ancient one, and exists in the folklore and superstitions of many cultures.
Myself, this is how I always understood the reasoning for events. Sure, Voldemort could have easily broken in and stolen the prophecy or found a super secret way to finesse himself inside, but it would have done him no good since Harry was the only one who could retrieve the prophecy.
In Chapter Five of Deathly Hallows Lupin states that the Polyjuice Potion is only meant for humans:
"You're half-giant," said Lupin, looking up at Hagrid. "The Polyjuice Potion is designed for human use only."
Alternatively, it might not have worked anyway because if he transformed into someone else he might no longer be considered "Voldemort" and thus would not be one of the people that the prophecy is about, and the regular protections would stop him. (That would depend on whether the prophecy protections can be fooled by Polyjuice Potion.)