I just had a glimpse... OF HELL!
Calling up some site backups to search for a missing node, one backup of my Drupalace.com database generated a huge listing of errors when I loaded it into a dummy site. A quick look showed a MASSIVE error list related to trackback comments. (Dopey me, I was so stunned by the sheer number that I failed to take note of what exactly the error messages were saying.)
It didn't take this Ace long to see what was going on: my site was logging zillions of spammy trackback links - Drugs! Russian Chicks! Loan Approvals! As I have trackback moderation turned on, they were filling the moderation queue, not my pages, and I had been blissfully unaware of the growing mass.
How many trackback spams are we talking here? A quick visit to the approval queue in /admin/content/trackback showed a page packed full of 50 messages... no, wait, there were pager links to more pages... let me click to the last page...
Criminy Pete! Almost 1000 pages of trackbacks! Nearly 50,000 items!
I marched right over to the Modules form, disabled the Trackback module, and then via myPhpAdmin emptied the overstuffed table full of trackback comments. It made a visible difference: I first exported my site (always smart before playing with tables!), which was 4.1MB as a .zip archive. Then I emptied the trackback comments, and exported again: 1.7MB. Almost 2/3 of my site database consisted of trackback spam!
I'm sure trackbacks are useful, but to be honest I've never seen a big effect or need. I probably erased some valid trackbacks in my purge. Let them go; having discovered the extreme spamocity of trackbacks, I'm laying off of them cold-turkey.