As mentioned recently, I digitally sign my email. Why? After all, by signing my email I vouch for what I say (no disowning it later) and I relinquish my anonymity. Wouldn't it be better to use some anonymizing service, not attach my name to what I say, not sign my mail, etc.?
Well, no. Here's why:
Anonymity doesn't matter. I'm proud of who I am, so I don't feel the need to be some anonymous shmoo on the Internet. (If I lived in a repressive society, I might feel differently, but in free societies anonymity isn't important, at least to me.)
Trust matters. As Francis Fukuyama has argued, societies with a high radius of trust (i.e., in which people readily associate with those far from their own kinship groups) are more successful. I want the Internet to be successful and to be a high-radius-of-trust extension of real life. One electronic marker of whether someone is trustworthy is whether they sign their email.
Identity matters. To me, trust and identity go hand-in-hand. I don't particularly trust anonymous people -- I need to know who they are (or at least that they have a stable association with a key, a cert, a mailing list, a blog, a website, etc.). When I receive email from someone with only a nickname, I immediately discount what that person says. It goes all the way back to Aristotle's Rhetoric: one is more likely to listen to or believe the argument of one who has a good reputation (and reputation is impossible without identity).
In the early days of the Internet (when it was almost solely an academic environment), access was tied to identity, users were not anonymous, and the medium was a high-trust microcosm of society. Today, Internet users have extremely weak identity (if any), email addresses are easily forged, no one knows who anyone is, and the result is a low-trust electronic slum. Use of digital signatures, server certificates, and the like is a way to help build a higher-trust Internet (or alternative community within the Internet). Those who use digital signatures today are like urban pioneers in a bad neighborhood. It's not clear if we're going to overcome the forces of darkness. But at least we're trying.