As the founder and leading administrator on a small forum meant for dedicated history/archaeological studies, I can tell you it's a good rule. I once had a new member pass muster and immediately start spamming dozens of threads with photographs featuring child pornography.
Another administrator in his haste began deleting all of the threads immediately (and I cannot blame him) instead of trying to delete the individual posts. As we are on a free server and have little ability to control the inner workings of our forum, these threads were all lost to us despite an attempt to retrieve them from the host site.
Such things suck, and happen rarely, but they do happen. The 10 post rule is a pretty simple and effective way to prevent it.
-Gregory