I think (since PA isn't just a suit of armor, but actually something much more) that they should need specialized stations to be weared.
I mean, something like "Power Armor Maintence Station", to put on your PA, and again to put it off. This way it would be a "permanent" suit of armor (like a perk that boosts stats and changes character sprite), that cannot be kept after being killed, cannot be looted (no way you can take some kind of... metal skin off a corpse - feel free to try if you want, but i don't think so -)
Instead of standard deterioration, it would have to get minor maintence, except in particular events (cannot think of any right now).
OF COURSE when you have a power armor on on you cannot put anything over it (again, feel free to try)
And, OF COURSE you would need to get access to one of those facilities, and also somehow get an armor itself.
Still, as someone else has stated, Power Armor isn't there yet for balance reason, like alot of other "overpowered" equipment.