Although a similar topic has already been brought up, I would like to point out that this is completely different. (you'll get it in a minute)
When a user signs up, says hello to everyone, and just dissapears, he or she leaves a user account. This account just sits there, doing nothing. Not particularly taking up much space, but it is preventing other users from using that name, other users who might intend on staying longer.
Now this poses a bit of a problem. I've seen some great names that I actually tried to use when I signed up. I ended up compromising with a name I have grown to hate. (although I thought it was novel at the time)
For instance, Meech, I tried to sign up for that but it was already taken...
Yeah so... yeah.
Just a thought.