I look at it like this:
- When the QB has time, Kaaya is obviously the better QB than Rosier.
- When the QB is under pressure, Rosier probably has a better chance of making a play, but we haven't seen enough to know Rosier would actually make that play
- The QB probably has time to throw 70% of the time, and is under pressure 30% of the time
By benching Kaaya, you'd be taking out the guy who's better 70% of the time, to roll the dice with a guy who might be better 30% of the time. Kaaya has his limitations, but you have to play the guy that gives you the best chance to win.