I think that the philosophy behind "don't sync on delete" is that people often handle their unread mail in two passes: 1) go through deleting the obvious junk, then 2) handle what's left which may involve reading, responding, and deleting). In such a scenario, forcing a sync between items in phase could be wasteful. I believe that responding to a message also forces a sync so any response may cause the deletion of items to get back to the server.