I see your point that existing shields would cover the A6 and A7 pins (either making them hard to use or inaccessable), but remember two things. First, these are development boards and the pin layout can still be modified slightly for the commercially sold boards. Second and most importantly, the Uno only has A0 through A5 so any Uno-compatible shield would only use 6 "analog" pins. Therefore, at worst an Uno shield would cost the use of 2 analog pins, but should still function all the same.