MMA is not easy to learn and for many military job discriptiions would be useless and a waste of money. Now for special ops infiltration units yes it could be helpful.
I've seen a lot of people get moved to doing jobs outside their MOS training once they are in the combat zone areas. No martial art is "easy" to learn, at least not easy to learn and be good at doing. Saying it would be useless is kind of like saying that teaching people to shoot for certain MOS is a waste of money and useless since they will unlikely be a situation that would require them to do so. Of course who knows what the future holds, and being prepared isn't a bad thing.