OK, using your analogy.....
I've watched them do this 50 times. But now, I'm supposed to go out there and do it for the first time, under the most difficult circumstances imaginable. Do you think you'll be able to make any sort of accurate assessment of my skills or ability? More likely, I'll just fall on my face, and nothing will be accomplished, because the next time the thing has to be done, the person I've been watching is just going to do it anyway.
Same thing with backups. IMO, it is a pointless excercise to put them out there in a situation that they stand a great chance of failing at. It's going to be the worst conditions imaginable, because they haven't taken starter reps during the week, and the other team is gonna tee off. It's not like Parcells had time to prepare either backup during the week this year. He had to prepare the team like they were going to be fighting to get in the playoffs.
No preparation+nothing to play for+first NFL start= Pointless disaster that proves nothing conclusively either way. Parcells knows what both QBs can do. He doesn't need to put them in a game, to satisfy our curiosity, to make a judgement on either of them.