I guess, but will the troops have to qualify on each round? Each will have different firing characteristics and a different point of impact at different ranges. If you wanted to simply spray and pray in a firefight why not just pick up a dropped enemy weapon or pick it up off a dead enemy and shoot? Seems unnecessary to me.
Now, I can see having the capability to shift between 5.56 and 7.62 x 51 with a barrel change to go along. That would shift the effort from an assault rifle in the 5.56 to a possible designated marksman in the heavier load and longer barrel. That makes up for the middle ground between the rifleman and the sniper.
The rifleman covers the area from the muzzle out to about 300 meters. The sniper engages at 700 to 1000+ meters. The new category of "designated marksman" (born of combat necessity) covers the ranges from between 300 and 700 meters. That is the perfect operating range for the .308 Winchester with only minimal training.
Remember, this thing started off life as a SOCOM weapon. Resupply of an ODA can be a tricky business depending on their circumstances. So the ability to use indigenous weapons and ammo can be of a great benefit to such soldiers.
You do bring up a good point about using the opposition's weapons, but again remember the characteristics of the AK-47. While it is dependable, and fires under extreme conditions that the soldier encounters in the field, it isn't a very accurate weapon. This is due to the loose tolerances that are used to achieve the advantage of dependability. Designated marksman capability would deteriorate, let alone sniper capability.
Sniper capability is going to be attenuated anyway under those conditions in that the availability of match grade ammo used in such weapons might not exist.
This is a case of the military planning for every situation. Perhaps they got it right this time.
As a stop gap, i don't know why the Army didn't go back to the M-16A2 for Afghanistan. It has a lot longer range than M-4. The M-4 worked well enough for Iraq, but combat in Afghanistan is a different situation.
My boy used an M-25 when he wasn't stuck on an M-240 or Ma Duce aboard an MRAP on convoys. He got his Bronze Star doing things best not remembered with an M-25