IIRC this has been suggested before and the fear was that the template would remain untouched and new text added at the beginning or at the end. Maybe that could be addressed if the focus was on the textarea and the template text was already selected, so anyone typing without reading will get rid of the template text at once. Other monks suggested to detect whether the template was still there and don't let the author create the node until changed.
In any case, I support this idea because in case the author left the template untouched it will be obvious after posting and hopefully will push the author to edit the node again.