Originally Posted by Ygle
(I've been using my stoneware and pyrex stuff anyway, but not sure if it's damaging/degrading them in someway?)
I doubt its degrading them, but the uneven and direct heat could potentially cause them to break, hence the warning. I'm pretty sure the uneven heat is a bigger problem with smaller toaster ovens than with large ones. Mine is mid-sized and I've never had a problem, but a friend of mine has one that is quite small and she has had pyrex break in it before.
I do use pyrex and stoneware in my toaster oven as well. I also use anything stainless steel. I have some camping bowls and plates (by MSR) made to be able to go directly on a fire that I use in the toaster oven for small portions or to heat a bowl/plate of food.