Yes, especially if one has parents who barge in without knocking. 8( My mom never quite understood the concept of knocking. Oh, she knocked, but she would knock as she was opening the door. So if I was in various states of undress or something, there was never a chance to say "hey don't come in yet!" or to move out of the line of sight of the door. :| Even just that, nevermind a lock, would've been nice.
Well, I did have a lock but it didn't work. :( Mostly everyone but mom respected that. *sigh*
But I do believe a lock is fine. If it's a lock with a key, both the room owner and the parents should have a copy "just in case" but... if you're having privacy issues, that's not going to be solved by just putting a lock on your door. That's something you've gotta sit down and talk through with your parents. The lock would mostly just be there to keep siblings out and the like. :s