Thanks for getting back to me so quickly Sam 
How would post_id not work for this scenario? It would seem that if 2 is deleted, the third post would become the second post, but its post_number does not change accordingly and is now wrong. Post_id would behave the same way in this scenario, except that it would not wrongfully identify what is now the second post in the thread as the third.
Agreed that it is a prettier URL.