I kind of always liked the idea of the 6.8. More effective at dropping targets at range, and close. Not a huge amount more heavy, unlike the .308. And there isn't too much recoil issues.
I have to say, though, that the fragmentation of ammunition in our nation's military is kind of silly. The .308 round is a good all around cartridge, and it is already implemented in some of our nations finest arms. Would make sense to me to try and put more effort into usable, durable, accurate, and light weight .308 firearms. Just to start a little somethin': I think that fully automatic weapons are a waste. When you can just get a mag of 30 rounds and yank em off as fast as possible, it doesn't make sense to have that 30 finger twitches automated. Unless we are talking about cover fire. You could make the argument that every man should be adaptable, and the only way to do that is to make their weapons adaptable.
Not to mention the .308 is also a fantastic hunting round. More .308 use in the military would mean better R&D for .308 firearms to us civis. I would love to see better hunting arms with military flare in the hands of the average hunter.
All this debate over which cartridge makes me kind of giggle. It also makes me wonder why our military even uses the M4 at all. Such a great gun for hunting and recreation, but really not so good as a weapon. On the other hand, a FAL or G3 are perfect weapons. Strong, durable, accurate, reliable, and generally very useful in combat, just a tad too heavy. Oh well...