Here's an example question, which passes the What kind of questions should I not ask here? test. For me, it was really a matter of curiosity, because I don't intend to ever use GTK+ 1.x. But that there's been at least 6 votes on the thread and dozens of views indicates there was actually was some interest.
Whether you have a practical interest in the answer or only idle curiosity shouldn't matter: after all, if you didn't say, we wouldn't know.
Note that a high number of votes doesn't indicate a good question. (Search for things like “favorite cartoon” on Meta.SO.)
So long as your question isn't hypothetical in a meaningless "I'm bored, entertain me" way -- that is, it is interesting to other experienced users, that's probably fine.
Sure, if it's a real, answerable question. It may be only curiosity to you, but turn out to be practical for someone else. In fact, it may turn out that the reason you were curious comes around at you later some time, and you're glad you learned the answer.
My guess is that the curiosity-drive is a major reason people end up hanging out on these sites. Sure, practical answers are nice, but that doesn't explain the really invested Stack Exchange experts. Since that's good for the site, it should be encouraged.
So long as it would otherwise be ontopic, not too subjective, or too localized. I don't even care if you already know the answer.