Messes go further than just the heating element. Things splatter, run over. Even with controls on the face of the range (not always practical) you may need to clean them from messes.

Its definitely easy to clean my entire range being a flat piece of glass, and personally I've never had any problems with sensitivity on the buttons. Plus, that whole cook top is then still useful as a counter top as even the glass is only slightly raised over the rest of the counter surface. In my experience the glass top range I have has been wonderful, and I was originally planning on tearing it out and putting in a gas unit. I've since second guessed those plans and will probably keep the range for a while longer, no knobs has actually been pretty nice.