What kass said. But also, while I certainly don't mean to minimize the importance of the safety issues around pseudonymity, one other issue that's often overlooked is that a robust culture of pseudonymity can improve the quality of discussion, and often does. Where no one can be sure of the (logically irrelevant) RL privileges that the person behind a persistent pseud might bring to a debate, participants have to focus on what was said, and come to grips with that, rather than dismiss some speakers as being unworthy of their full notice and consideration.

Which, I often think, is why authoritarians and those with authoritarian tendencies hate it. And reason enough to support it right there -- even if you don't go quite so far as I do, and find yourself instantly and automatically losing all respect for anyone who argues in favor of mandatory use of RL names.