(Axel Boldt axelboldt@yahoo.com):
If I remember correctly, we initially implemented nicknames to accomodate users with offensive user names. This issue is now moot, since by consensus offensive user names are not allowed anymore.
Actually, I implemented the feature just because I wanted it. I still think it's useful--I want my user page to be headed by a complete name, but using that long name in attributing comments seems like overkill. Plus, it gives people the ability to personalize a bit while not hiding behind a psudonym entirely.
Like many features of the software, I think this one just gives us a greater opportunity to judge the character of the user: if someone changes nicknames to deliberately obscure the attribution of comments, then we know how seriously to take those comments.
If users seriously abuse any feature of the software and disrupt our goals, we can consider taking action. But I don't see this particular feature as more abusable than any other.