Chopped onions change smell after they've been sitting exposed to air for a while, and not for the better. I'll happily prep my own onions, even if it brings tears to my eyes.

Sweet potatoes turn brown exposed to air, sometimes even in the span of time from prepping the first to prepping the last before they go into the pot all together.

I would venture that most things do not improve with exposure to air once cut up. I would also tend to agree that just sending unprepped ingredients doesn't save any time, but I think I'm willing to take more time to get a better meal.