I disagree. You play whomever gives you the best chance to win, period. It's hard enough to win games as it is. Having a revolving door at your most critical position is a recipe for disaster, especially if one guy hasn't earned it yet. That said, if there is negligible drop off between the two, I'd say rotate and stick with the hot hand.I agree, that the better QB should get the majority of the snaps. Like 60/40 or 65/35, not 90/10 with the 10 being in mop up or hopeless situations.
There is no perfect solution, only the least worst which provides for the best long range outcome. In 2012, it would have been better to play Vad more since TW was leaving. In 2013, it would have been better to play JT more since Vad left. I have no idea about 2014 and 2015 except that they won't go according to script.
Another good use of the backup is when the starter completely falls apart on the field and loses confidence. But this just goes back to using the guy that gives you the best chance to win.