Thats a really simplistic and, quite honestly, poorly thought out argument. You mean to say whenever the power fist has power running through it your hand is stuck in a death grip of rage? You have to turn it off before you can climb a ladder, grab a ledge, grapple an opponent, or pick anything up? Otherwise your hand is stuck in a closed vice grip?
Sorry, no. That makes no sense at all. Besides the fact your fingers don't need to be super strong to punch someone with super strength, once you turn off the power you wouldn't even be able to open the fingers. Unless of course you mutate your fist to be giant sized, power fists use a waldo system. They aren't gloves. No power = no movement at all.
Using a waldo system, which includes force feedback, a marine would more than easily be able to regulate the amount of force and strength he is using at any point.
So you contend a marine who takes a power fist is not allowed to use that hand at all as long as the power fist is attached? What's the point of having a strength enhancing hand if it doesn't enhance your strength? What about marines tearing apart walls or other items with power fists? Do you really insist its just a weak version of a thunderhammer (at the same cost)? It doubles your strength (2 x 1d10 unarmed damage =/= 2d10? wait, what?) or adds to your unnatural strength… except when you want to use it for strength? Nope, none of that makes sense.