Which even mages agree is doable-- but their problem is that those diminishing returns are shared by other, controllable, freezing effects-- Frost Nova and the Water Elemental's Freeze. Basically, an accidental Frostbite proc, which can't be controlled by the player, could make the opponent immune to controlled freezes. And considering that Frostbite, when talented out, procs a lot, that puts a damper on regular frost mage abilities.
The good news, however, for mages is that the problem is being addressed. Tseric says that Frostbite's diminishing returns are meant to only affect itself, not other Freezing spells. They're working, he says, on fixing it.
Which is great (well, not so much for people fighting mages, but for mages it is). But this is the second big mage "change" we've seen on the PTR that turned out to be a bug. What's going on over there at Blizzard in the mage department?