While it makes sense for fists to help with attack rolls, I am against it for balance reasons. You already get added stress on the weapon with the power.
That said, if someone wanted to homebrew a rule that:
The +2 added to the weapon value (or +3 etc) are added to weapons or fists skill instead, that would fly too.
In the end, on a successful attack you'd be doing the same amount of damage. And I know that some people would balk at being able to hit easier... but keep in mind that stress is an abstract thing.
If you've ever fought with swords or fists, you know that fighting a stronger person... it hurts even to block. It causes stress to avoid stress.
So if my players made a convincing argument for a house rule like this, I would probably allow it. It would really make the inhumanly strong character feel more deadly too.