When a team waives someone, does it take a certain amount of time for that player to clear waivers? If that's the case, it may be why Danny had to trade Cassell. If it takes something like 48 hours for a waiver request to go through, Danny may have needed to deal Cassell today so that he could be down to 14 players in 2 days at the deadline.
That could explain why he just didn't waive Cassell, Roy's financial reason aside.