I once had someone else in a MUD I played go to some other site and make a character using the name I'd come up with for my character. This person was a dweeb of the sort that flashes and is gone, leaving behind a bad taste in everyone's mouth; and I'm sure whatever he did with my name is long gone. But it still irks me to think about it.
It's occurred to me that the right of each person to choose what they are called seems like a fundamental right to me. It seems very petty when you talk about it, though. If someone chooses to call you by a nickname you wouldn't choose for yourself, that's a mild irritation at most, right? A minor issue at worst. And yet somehow it seems to me that it sits right on the border of something very big.
In a way, choosing your name and being able to decide when and how it's used is a symbol -- but more than a symbol in some way I can't put into words -- of deciding your own identity. Consider a comparison to those who choose to live in a different gender than the one into which they were born; it's an important sign of respect and courtesy that we allow them to choose what pronouns we use when we refer to them. In a way, that's the same thing.
I feel like I could almost write a short thesis about why the right to object to someone borrowing your name, or giving you a nickname you don't like, is no minor petty quibble but a major slight. But I can't quite put it together: I can't quite figure out the arguments that would be needed to bridge the gap between the idea that nicknames are a light-hearted minor thing, and that they're part of letting a person choose his own identity.