Well, if we use power-armored soldiers, I doubt we'll be using them in large blobs due to cost. And someone will find a way to counter them cheaply and effectively, and the best way to do that is to just use bigger guns. NGTM-1 thinks being able to stop anything short of .50 caliber rounds will help, except it is cheaper to have a bunch of .50's lying around in case of power armor than it is to have the power armor. Armor really only works if it forces your opponent to have a major shift in tactics or to develop new weapons, and having a suit of armor that needs a simple pintle-mounted .50 to penetrate it may not be that useful. At that point you're better off with dragonskin armor for cost-effectiveness, especially since I've seen a video that had a dragonskin vest be literally on top of an exploding frag grenade and not have any shrapnel penetrate through the armor.
Very silly!
'A simple pintle-minted .50'. Not simple. Not cheap. Not common. Not...man-portable.
You cannot pretend that it's as easy to acquire, transport and equip a bunch of fifties as it is a bunch of AKs or RPGs.
What you're arguing is that armor isn't worth having if there's a weapon that can defeat it. Yet body armor in Iraq has rendered the casualty rate so ridiculously low - even in cases of direct mortar fire - that medical science has had to open up whole new fields to deal with injuries to people who would otherwise have been dead. And this body armor can be defeated by average infantry weapons.
Armor that can defeat everything up to a .50 would be a massive change.
Not to mention you're ignoring casualties due to things that probably take down far more soldiers than bullets or explosives: heatstroke, exhaustion...power armor can prevent these while enhancing mobility.
Let me go back and call out the most ridiculous statement in there:
Armor really only works if it forces your opponent to have a major shift in tactics or to develop new weapons,
The combat armor used by troops in Iraq works. Unquestionably it works.
It has not forced or required opponents to shift their tactics or develop new weapons. They still use AKs, RPGs and mortars. They still kill American soldiers with them.
They just don't do it as well, or as often.
The armor works.
Yeah, but then we stop using small-arms and go to assault rifles firing mass-reactive miniature RPG's.
Still a very different ballgame from fragmentation weapons.
Imagine that you're playing an RTS game where you counter large blobs of small units with splash damage weapons...and suddenly splash damage stops working.
Then you start countering with directed explosives and volume of fire. 
Also absurd. What you're suggesting is a completely different matter from fragmentation weapons - more difficult, requiring more equipment, more precision, more training, line of sight, and more people.
Infantry warfare would need to be totally redefined. COIN and asymmetrical warfare would also change, because your average insurgent group can't manage what you're suggesting.