I'm purchasing a Corningware 4 quart stoneware casserole for a friend. I am very surprised to see it stated that this product should NOT be used on the cooktop. Now, I've used one on the cooktop for 40 years - soups, stews, pot roasts, pasta, etc. No problems. So, um, what's the deal? Have I been extraordinarily lucky? Now I can imagine that if you let the contents dry out, temps could get dangerously high. But as long as there is water in it, how can there be a problem? Just curious if anyone has seen one of these units break on the cooktop, and what you were doing when it did.