I agree with you. I think it is a fine question. Why are people so defensive about it? One of the arguments is that people can prepare for it. Okay, prepare for it. You can tell if someone has prepared for the question or not by how much they think when they answer it. If they've prepared for it, the answer better be a good one.
Furthermore, if someone gets defensive when the question is asked, then that tells you something about that person. It tells you how they operate in tough situations. It tells you if they are honest. Honesty matters a lot in a stressful situation, because if you are trying to fix something and someone who messed up isn't saying they messed up then it is going to be a lot harder to fix it. Once you've discovered the error someone made and the truth comes out, now you're stuck with someone who messed up and lied to cover it up. An "oh... yeah, I might have flipped that switch..." will go a long way toward making everything right.
Furthermore, if someone gets defensive when the question is asked, then that tells you something about that person. It tells you how they operate in tough situations. It tells you if they are honest. Honesty matters a lot in a stressful situation, because if you are trying to fix something and someone who messed up isn't saying they messed up then it is going to be a lot harder to fix it. Once you've discovered the error someone made and the truth comes out, now you're stuck with someone who messed up and lied to cover it up. An "oh... yeah, I might have flipped that switch..." will go a long way toward making everything right.