Yeah that was mentioned earlier. The whole argument that two guys have to carry the wounded guy off the field, taking 3 guys out of action, whereas the dead lie as they are, so killing only takes one person out of action.
The other argument I've heard is that the 5.56 was intended to be a suppressing fire round only. Basically, since it's smaller and lighter, SAW gunners can carry more of it. It's not meant to be deadly, it's just meant to keep the enemies' heads down, while riflemen armed with .308 battle rifles can get into position to actually kill them.