Ninety-nine percent of posts are never deleted here, because we want users to be able to read their and others' old posts. Things can be learned from them, and they can be entertaining. And I'm sure Nate has other obscure reasons, as well.
I don't delete posts on either of my sites for similar reasons. Also, I firmly believe in "preserving the evidence" if someone does something bad.
If someone does do something naughty naughty on there, rather than editing it out the forum will automatically put it into the, uh, "Gunship Can" (I love the censors here XD) when it gets deleted.