I'd like to add that identd has a third (or second, depends on how you look at it) use:
Many MUDs have banned multiplaying (player characters-to-human beings ratio != 1, that is, either one person has many characters or many persons share one character). Now, identd is obviously quite handy in preventing it. I don't know how other muds work it, but Batmud always asks for identd information (next time you login watch your /var/log/messages ... cave.cs.hut.fi should call in shortly), but normally ignores it. However, if a player is suspected of multiple-character-crime, it is checked whether other players might be sometimes logged-in from the same host. Now, if the host provides identd information, it can be used to determine that other players from same host are not the same person. If there is no identd response, players are assumed to be one person are are all removed.
How do I know this? Simple. I got hit by it. Someone from my shell provider multiplayed, I had left my character to idle under screen, and the machine's identd was broken... bang.
I'd also like to add that there are several fake-identds (sends out whatever information you want ) out there, so the real usability of identd limits to servers with clean and/or ignorant roots.