I always pictured PA as sizeable things, not something that could easily get through a doorway (Modern day door way in Amerika) due to the width problem, at least.
As such, I always pictured them as some what unwieldy in certain environments, like urban (House to House), or ship boarding.
Sure, one could break down the door with your Power (Thirst/Armor) strength or something, or just shimmy on through, but the former would make noise and possible time, while the latter would... Probably do the same, actually.
PLUS, PAs aren't the most stealthy of things usually, so normal infantry has that perk.