For one thing, they make it so that the oven takes forever to heat up. I left a skillet and a grill pan on the bottom shelf when I was making banana "snack cake" and later on in the week I made magic bars for the karate picnic. Yes, I made this mistake twice.
The edges were the only cooked pieces on the banana cake, I was able to salvage this much and throw the rest away. The boys loved it, though. Will asked for a taste, and then took the piece from me with a big smile and a "Mine!" :) It's going to be the first recipe I write into The Notebook in a while.
The magic bars edges were destroyed. Not only because of the cast iron roommates hogging space in the oven, but because I thought that the first step of melting butter in the pan would be enough to grease it. Yeah.....not so much for the sides. This is how much I had to throw out, and I despise throwing out food.
So....lessons learned and shared from me, so you don't have to learn them for yourself.