Considering modern combat (not just all the guerilla stuff, but the rest as well) consists about as much of urban combat as it does long distance shoot-outs, they'd be screwed if they'd have to actually attack a place.
They'll also have trouble on defence in terrain where the enemy can get to within, say, 300m. And that's ignoring the problems posed by aircraft, tanks, artillery, mechanised infantry, etc.
Snipers are very specialised solders, as has already been mentioned, and they are not appropriate for many circumstances. I think this idea is impractical.
MacCaulay makes the point that this isn't really intended to be a practical suggestion, and instead it's about the effects on the soldiers themselves of having to see their enemies demise. I haven't personally seen any research into whether snipers suffer greater PTSD rates than other arms - I'm not saying it doesn't exist, I just haven't bothered looking for it. But that strikes me as a good place to start, because the difference between shooting someone with a .50 cal bullet and landing a propelled grenade at their feet seems academic in most respects.