If we are talking about military MRE's (really the only ones I know anything about) then yes, they will take quite a bit of indirect heat. If they touch the flames though or get too close, the bag will melt, so it is about of trial and error to you feel out the right distance.
Nothing wrong with eating them cold, and there have been many a day when we were humping that they were eaten while walking, but, given the chance, I was all about making something tasty. I mean, I have lived months at a time with nothing but MRE's, and it gets old... ESPECIALLY cold. My point is I do not ever want to have to go back to that if I can help it, so therefore I would rather augment the meal, rather than "rely" on it alone. Mix up a little cobbler, or get a bag of ramen to add to an entree, or maybe even some fresh eggs.
Makes sitting in some armpit of the world just a little more tolerable if only for a little while... but then I am a medic, and a foodie, so my guys always came by my "hasty fighting position" to see whats cookin!