halacha - Is it prohibited to derive benefit from a mixture of milk and meat if the animal was improperly slaughtered?

Is it prohibited to derive benefit from a mixture of milk and meat if the animal was improperly slaughtered? Assume the animal slaughtered, like the milk-giving animal, is kosher, but the slaughter did not meet standards of kashrus. Would one be permitted to derive benefit from the mixture (i.e. feeding it to a pet)?

It seems that Rambam writes about this in his commentary to the Mishna (Keritut 3:4), but that his opinion is not widely considered normative. Why not?