The big problem I have with the current system isn't actually the Daily use, it's the X per Milestone use. Why shouldn't a player be allowed to use all his items in one fateful blow? Or for that matter, four different items on four different turns?
If "Bag of Items" got out of hand, I would simply implement a one Daily/Slot rule. My general experience (and I'm aware this is anecdotal) is that most players either want to the play the game fairly*, or rapidly get sick of switching out items. Abuses aren't generally because of a flaw in the system, but because of a problem player. In which case, the correct thing to do is talk with the player, not limit the system.
*Fair is a matter of perspective, I suppose.