It's not the bodies that create the heat though, is it? If it were gas given off from bodies, that would be a different thing, it is simply using the extra energy that is wasted.
If we aren't prepared to look at such schemes, our fuel will run out even quicker than expected. (Not long, sadly)
Incidentally, I live nearby, and although DD wasn't created in the crematorium but a neighbouring one, as a bereaved parent I would have absolutely no qualms about heat from her cremation being used for the greater good.