Everytime I wonder why they use the 5.56 that fact always comes to mind. There are other factors to consider aside from the ballistics. I suppose this is why we see the same arguments between .45 vs the 9MM for close quarters combat. Sure the .45 is superior for stopping power, but the 9MM can be used for both MP5s and sidearms. That allows them to carry the same ammo for two weapons. Of course they could use .45 subguns and pistols, or even .40s, but you get the idea. Logistics are an important consideration, and weight is one of those considerations, as you said.
The SOG troops that "didn't" go into Cambodia and Laos carried the super-short CAR-15s with at least 12 30rd mags plus all their gear. Try that with anything chambered in a 30 cal rd.