officially hexamine is an explosive compound originally used in mines and torpedoes which could be why its not doled out willy nilly. plus you dont get issued packs or cookers every day. half the time we struggled to get enough and relied on old hoarded stock or doubling up. they are supposed to issue one pack a day and the cookers were only meant to be given out every third day. most of the time we only got half a pack a day as the required amounts didn't make it out to us.
the american mre heaters are okay for warming a meal but rubbish for anything else and there are lots of complaints about the lack of a heat tab now that trioxane isn't made anymore. mre's come with coffee mix but nothing to make it with so they now give out bags which you fill with water and put in a heater but it wont boil the water.
we used to save them up for quiet meals or bottle bombs when there was shortage of blue grenades during fibua or fish.
none of the crusader kit is general issue but the mugs are becoming commonplace and from what I hear the heaters are cropping up now.
I hated hexi as it was a bugger to light but they seem to have improved the binding agent now so they are less waxy and take a match better but crumble too much if you break them up - it could just be me though. I allways packed it for emergencies and still do.