Personally, I don't change my name (Although I went through many early on.) and it wouldn't affect me, I stuck with _matt_ since early 04 and Matt after the 'username clean up.' But I guess a good reason of why it would be 'important' to keep the ability to change around is special occasions (Prime example: Obviously right now I am DeathMuffin, in honor of the birth of the muffin crew 2 years ago.) and uncertainty upon registering. What I'm saying is I started out with "mack8354", and looking back it's a crappy name and had I been forced to stick with it this account wouldn't be very active today, sure I may have gone ahead and made a new account, but most newer users most likely wouldn't have the determination to do so. Moving right along, It's definitely not necessary to change your name during the holidays, but it causes the userbase to be more interactive and have more fun, that's about it.
Agggh, I don't really know. As long as people do not abuse the ability I don't suppose it would make the site too sloppy, but then again taking it away wouldn't really affect me personally so I may be a bit biased.