Either Sanchez is a pretty good candidate for your QB3, in which case there's no point in developing Showers for that role if he's not better than what's available on the street at the last minute, or Sanchez is a bad candidate for your QB3, and Showers still wasn't able to beat him out despite knowing the offense and getting a heavy dose of reps in camp. Either way, how much does Jamiel have to develop just to get to a point where he can make the roster? Quite a bit.
If we hit on Dak, we don't need a third spot for a young developing guy anymore. We need a vet in that spot until Romo gets back, and then we don't need to waste a roster spot on a third QB at all.