The next major release of IPB should have more tools/utilities/tasks to remove obsolete records from the database. Some examples are:
#1 - delete contacts from the database of members who have been deleted
#2 - delete friends from the database of members who have been deleted
#3 - delete out-dated lost password requests
...

Also: delete orphaned posts/topics and PM-messages/PM-topics.

... caused by us running mySQL queries to delete PMs from spammers.

... you know what, a "Delete all PMs by this member" would be great too.

Bumping up this topic for consideration in future versions, especially #3. I found today I had nearly 1300 rows in my ibf_validating table from old lost password requests dating back 4 years. Something should clear those out after X number of days, similar to the task that clears out incomplete registrations.