I've actually been thinking about asking for one for a while (still haven't made up my mind yet). Probably not an entirely permanent one, but one to just give me time for my A levels which I been screwing up entirely revision-wise. That's why some users ask for it, because they need to prioritise something other than this site which takes up too much of their time (Ultrajoe, mscherbatskaya or however you spell her name and Kukul all asked for suspensions for that reason).
Generally people just get bored of the site and decided that they no longer wanted to take part in it. It's a shame when it happens but I'm glad to see that they're no longer trying to force themselves into posting still, even when they have nothing left to say. Sometimes they decide that they haven't finished after all and sometimes they don't. Either way it's a shame to see someone I like choose to be banned, but at least it was their choice, instead of being banned for being a dick.
Sven und EIN HUND said:
I can't remember any instances where people asked mods to ban them.
Max was going to ask for one before he was banned anyway. It's the main reason why he's glad to be banned.
EDIT: Fightgarr asked for one a while back before asking for it to be lifted again shortly afterwards.