One thing that I'm certain has been done from time to time is removing accounts that were never activated. That means they signed up, but never clicked the link they were emailed after signing up, and so never actually logged in for the first time.
I don't think we should delete accounts that aren't spam and haven't posted, IF those accounts have been used to log in before. Plenty of people just want to lurk from time to time. And maybe one day they will want to post, and they shouldn't have their account deleted just... well just because. I don't think our database is struggling to keep up with requests we are throwing at it, so a major purge isn't really something I think we need to be doing.