No one wants to be shot by either Caliber...
They each have thier strengths and weaknesses however. 5.56 is heavy enough for most situations a soldier will run into, however .308 has its uses.
The .308 is a little more reliable over 300 Yards in both lethality (Even the biggest .223 fans have to admit that outside of fragmentation range the .223 isn't doing anything better then .30 cal) and consistancy.
There are heavier rounds for the m16 that are fine and dandy at longer ranges, but they are not general issue.
Another issue is penetration, the .308 performs better firing through light stone walls, car windows, building frames etc.
Now all this is not to say that the .308 supplants the .223, but having a battle rifle really augments the .223 very nicely.
It all boils down to helping our guys win, and its just damn handy to have an m14 around in Iraq and Afghanistan at the moment..