Other forums which allow anonymous postings sometimes have the possibility to enter a guest name. I like that because it makes discussions easier to follow, even in tree structures.

Of course, this has the disadvantage that you can enter any guest name, but guests are clearly distinguishable from signed in users, so that's not a big problem.

At perl-community.de for example, where I am a moderator, I see the typical process: someone asks a question as a guest, and in many cases when they get answers then they decide to sign in to join the discussion as a regular user. Or they sign in when they realize they have another question a week later. So I think guest postings encourage users to post a question. I myself don't like it if I have to sign in to a forum where I have no idea if I'll get an answer at all. There are already too many websites that have my email (and often my unencrypted password, too).

The most common reason against guest postings I hear from people is that you have no control, and people can post spam. But if moderators/experienced users have to approve guest postings this reason goes away.