So Joshua Kaufman mentions [Or not, he is giving the 410 message to a bunch of his stuff - link removed 03/25/2004] that he doesn't see a lot of TrackBacking going on.
Having just upgraded to MT2.21, my theory is that it's due to two things: 1) It's a cool idea, but not everyone gets it. I don't think it was initially explained as well as it could have been. Joshua does a good job of explaining, actually. 2) It took me more trouble to get Trackback implemented than the actual upgrade from MT2.1 to MT2.21.
I suspect I'll put more details up about this at a later date, when I have more sleep.
Update: when Joshua says "Usually I simply forget to send the TrackBack ping..." it gets me. Maybe it's the hour, but that sets off usability bells in my head. I would think that there has to be a better way than cutting and pasting URLs around.
Plus, I assumed for some reason that my second ping would replace my first, since it was the same post. I often don't bother with excerpts, but with a TrackBack entry, it's more important. Now I have two entries floating around over on Joshua's TrackBack page. (And is it weird to link to a TrackBack page on the post that started it? Seems a bit circular somehow)
I'll refrain from a tedious anti-popup window rant and let my site speak for itself.