If you actually NEED a reason...

...not to inadvertently overcook-to-the-point-of-smoldering your poor, innocuous microwavable heating pad, then let the scary, China Syndrome meltiness of my cooler lid fill that requirement for you. Also --though not photographable-- the insane chemical stinkiness that infuses the very air and will probably lace any food I try to microwave in the future with pestiferous carcinacules.  I think my fat fingers hit an extra digit. The Curse of the Unrelentingly Spatulate!