I've seen at least one case where some spammer posted something blatantly spammy, got flagged, and was deleted. Unfortunately, in the interim some forum members replied to it (mostly commenting about how it was spammy) and so taking down the initial post didn't take down the thread. In particularly, it did not take down the super-spammy title complete with actual contact information.
Now ideally, people would learn and not post in response to spam threads, but it seems like in the case they do, it would be better to delete all of the posts in the thread, once the original post is shown to be utter spam. After all, we already purge posts that quote purged posts for other issues. Is there a reason that it's not moderator policy to do that for spam threads?