It is rare that a back up QB ever leads a team into or through the playoffs. The last I can remember was Foles.
A very good back-up gets a lot of money. If you have a Rogers or Stafford or Brady, the drop off to even a "good" back up QB is pretty large.
I think the era of the high priced backup for a championship caliber team is past. If you're already investing that much money for a starting star QB, you just can't invest another 7 - 10 mil in a back up. A game or two injury to a starting star QB is something you hope an adequate QB can get you through. But beyond that?
More, I think, the high quality back up QB is for competition to a proposed starter, and a backstop if that proposed starter falters.
