I think the relevant property is not anonymity, it's accountability. The argument against anonymity is that it will make people more accountable, but that's not always true. Rather, it's the way an online community structures itself.

For instance, here on HN we have the ability to be anonymous (on a sliding scale -- people can say who they are in their profiles, or be completely anonymous, or somewhere in between, posting contact info so that with effort they can be tied to their offline identity). But compared to many online communities, there's very little abuse here, and I'd argue that's because there's accountability -- there are clear standards, and those are enforced both by the moderators and by the community itself. Whether you post as yourself, fully anonymously, or somewhere in between, there's a community standard that hold you accountable for your posts.

I, personally, highly value the ability to have online identities somewhat separate from my offline one. I'm not trying to hide anything, I simply find it freeing to be able to enter a community without dragging around the weight of previous personal history, and to be able to establish an identity specific to that community I'm participating in. I always found this one of the amazing things about the internet, and I've been saddened to see that undermined. At the same time, I absolutely try to be accountable for what I say online, I expect others to hold me to that, and I hold others to that standard. None of this requires I know someone's "real" name, or that they know mine.