You already know from previous posts that this is a troll, and you don't need to reply.

No I don't, because these posts may just be coming from different AM's. I can't tell, because I can only evaluate the post based on its content, I don't know where it came from. If the IP address is displayed I can tell, thus my decision-making capability is improved. If the IP address of the posts is not the same I cannot make the reverse conclusion of course, I can still only judge based on content, but I'm no worse off than before, when no IP address was displayed. And I've made it a bit harder for the AM troll to ply his trade.

I don't know whether people would use this information enough to make it worthwhile, thus my proposal (as opposed to a request). I personally would, so I guess I was scratching my own itch. I've heard good things sometimes come from that :-).

Anonymonks monks having their IP addresses listed aren't really anonymous, are they?

That depends who you want to be anonymous to. Some random DSL/company/university IP is not going to tell the average PM user anything about the person behind it. If you're trying to be anonymous against a government agency or your employer (and in certain jurisdictions powerful copyright holders), then you're right, that anonymity is not there. But that is already the case now.

What's an unofficial "anonymous" login?

An alternative login which you use when you don't want your reply associated with your normal login (or your IP address, according to my proposal). It's sufficiently anonymous because it does not lead back to a real or virtual identity (except in the cases detailed above). Sorry for being unclear, I thought I'd talked about that enough in my other posts.

And there's nothing that keeps a troll from creating alternate logins, but it's that extra bit of effort which may be enough to stop him from posting. The entry barrier required to discourage trolls isn't usually very high.

...Let's equip all red cars with a siren, so we know he's coming

No, it's more like saying "Let's stick a unique and identifiable number on the front and back of every car, so if one breaks the speed limit we know where to send the ticket." Now wouldn't that be a really fascist and impractical thing to do ;-)?