They didn't break the game before. Why do they now? Is it because characters can use more than one daily item? They're still limited to three non purchased items. Not exactly what I would call game breaking. Right now the "rare" items are the same gear any character could buy at their leisure by the old way.
As others have said - I think the goal is for rares to be approaching artifact levels of power, and allowing PCs to do unique things that are only really acceptable in small, DM-controlled quantities.
Now, some of the Rares seen thus far haven't been overwhelming. But letting every player have their pick of multiple rares? I'm sorry, but that is just asking for disaster to happen.
I like the idea of 'pick 3 uncommons, but you can only buy commons'. Some areas remain kinda tricky (namely, consumables), but I think that otherwise handles the current environment as well as can be expected.