I don't think it takes two years to evaluate talent a the quarterback position. Sure - like the "it takes three years for a receiver to blossom" discussion, they'll improve. But they improve from either a good starting point to become very good, or a mediocre starting point to become...something more than mediocre.
Tom Brady, Ben Roethisberger, Russell Wilson - all example of QBs drafted with varying (from none with Tom to high with Ben) expectations to start quickly. All have improved as QBs tremendously since their starting jobs. Tom and Ben weren't sure fire HoF'ers their rookie years, but their skill, physically and mentally, as witnessed by the coaches showed they had a good base and they learned the game and became better.
Russell Wilson was drafted as the thrid string behind expected starter Matt Flynn and previous starter Tavaris Jackson (ugh). Admitedly, Wilson was given equal opportunity that preaseason to showcase his talents, but it was obvious enough that he had the BASELINE needed to be decent and improve. Not "bad but with potential."
Drafting a "bad but with potential" QB is dumb in the NFL. There is no time in the NFL these days to progress a bad QB to a possibly good one when snaps are limited and wins are needed.
We should draft good QBs. They may end up not good enough, but I think they can be evaluated and moved on from within two seasons. The problem becomes the money committed to them when you decide to move on - which is even more reason not to draft "bad with potential" that eats up cap space and a player spot on the roster.