I used to keep my spam folder in GMail turning over automatically, just allowing items to expire as they reached their 30-day limit, then it seemed everything got a lot worse and my spam volume jumped from 20 per month to over 50. It held there for a really long time, and I got sick of marking things as read every day (to keep the unread message count invisible) so I just deleted the lot one day. Suddenly, the volume of new messages dropped, so I can't help but think GMail bases its minimum delivery threshold on how many messages you manually delete from GMail, not how many spams you receive or how many automatically expire from your spambox. Now that I've thought about it, it does make a kind of sense from a certain point of view, but it's not how I assumed things would work.
Mokalus of Borg
PS - Spam is spam, deleted or not.
PPS - And I like to have a rough idea how much I'm receiving.