(posting as myself, not as ubermoderator)

I'm in favour of deleting spam accounts, as there's no point leaving them around. All the spambots that I've spotted have just been permabanned anyway, and I'd be surprised if they ever logged in more than once.

I don't think that inactive user accounts should be deleted, without at least giving them a chance to reactivate them. Unused accounts are another matter - if someone's signed up but never actually done anything with their account, then it's unlikely that they'll ever use it.

I'm against any account purge that'll delete the posts made by that account (unless all their posts are spam).