Why did Harry and Ron take Gilderoy Lockhart with them into the Chamber of Secrets?

Question

Unlike Hermione, Harry and Ron clearly understood that Gilderoy Lockhart was a useless blowhard who didn't know anything and had no useful skills (especially after coming to his office to tell him about the Basilisk and finding out he had no intentions of going to the Chamber and intended to run away; and didn't do any of the stuff in his books).

“What d’you want me to do?” said Lockhart weakly. “I don’t know where the Chamber of Secrets is. There’s nothing I can do.”
“You’re in luck,” said Harry, forcing Lockhart to his feet at wandpoint. “We think we know where it is. And what’s inside it. Let’s go.”
They marched Lockhart out of his office and down the nearest stairs, along the dark corridor where the messages shone on the wall, to the door of Moaning Myrtle’s bathroom.

So, why did they bother taking him with them to the Chamber of Secrets, instead of just stunning or petrifying him and leaving him around, either in his office or Myrtle's bathroom?

Interestingly, Lockhart himself asked that, but Harry didn't dignify him with an answer:

“Boys,” he said, his voice feeble. “Boys, what good will it do?”
Harry jabbed him in the back with his wand. Lockhart slid his legs into the pipe. “I really don’t think —” he started to say, but Ron gave him a push, and he slid out of sight.
Harry followed quickly. He lowered himself slowly into the pipe, then let go.

_{hat/tip Ikotica's amazing Harry Potter youtube reviews}.

Answer 1

I can think of two good reasons

He was too dangerous to be left alone

Harry and Ron initially went to Gilderoy Lockhart to get his help. Upon Lockhart's confession that he was, in fact, completely useless, the professor attempted to erase their memories.

“Awfully sorry, boys, but I’ll have to put a Memory Charm on you now. Can’t have you blabbing my secrets all over the place. I’d never sell another book —” Harry reached his wand just in time. Lockhart had barely raised his, when Harry bellowed, “Expelliarmus!” _{Harry Potter and the COS, ch. 16}

Harry and Ron then felt forced to disarm him, essentially making him a foe and their prisoner. They couldn't just leave him in the room alone at this point; they were forced to bring him along to prevent themselves from getting attacked again.

Meat shield

Force him to "kick the door in" and take the brunt of whatever nasty thing they'd be facing.

They marched Lockhart out of his office and down the nearest stairs, along the dark corridor where the messages shone on the wall, to the door of Moaning Myrtle’s bathroom. They sent Lockhart in first. Harry was pleased to see that he was shaking.

...

“Well, you hardly seem to need me,” said Lockhart, with a shadow of his old smile. “I’ll just —” He put his hand on the door knob, but Ron and Harry both pointed their wands at him. “You can go first,” Ron snarled

Answer 2

A couple of reasons come to mind.

1. He is an adult, and however stupid and bad at magic he is, it might be nice to have some adult presence (other than Tom Riddle)

2. He was told to do the job by Dumbledore. Ron and Harry, Gryffindors that they are, think he should do it.

3. Maybe they think they're in trouble for Disarming and overpowering a teacher (like when they triple-Disarmed Snape in PoA).

4. This is the key point, and I think the best. It was because they refused to let him get away. When they confronted him he was packing a trunk. He then confessed that none of his adventures were real and tried to modify Harry and Ron's memories; but they overpowered him. They won't let him get away until they can expose him as a scam, and why not let him go first into the Chamber? Here's a quote:

   “Well, you hardly seem to need me,” said Lockhart, with a shadow of his old smile. “I’ll just—”

   He put his hand on the door knob, but Ron and Harry both pointed their wands at him.

   “You can go first,” Ron snarled.

   They just don't want him to get away.

5. Basilisk fodder?

Answer 3

Gilderoy is still the only adult wizard around, Harry and Ron are still 12-year-old boys; they need all the help they can get. At the time, they believe him to be a braggart and a bit useless. They probably believe his books to be exaggerations, but they don't think he is actually dangerous to them. They don't know that the events in his books are complete fabrications, or about his skill with memory charms

Harry and Ron don't really empathise with Gilderoy here. They believe Gilderoy to be "on their side". They don't understand his fundamental cowardice, or his potential for evil. They think that if they can bring him to the chamber then he will start bravely; he just needs some encouragement. They were surprised that he was planning to run, so they had believed him to be basically decent. When it becomes apparent that he wants to avoid the chamber they still want him with them, as he is the only fully trained adult wizard around. As I said, they fail to understand Gilderoy, and that nearly gets them killed.