That's like saying "to qualify for a perfect score for fielding, a player would have to be able to do everything defensively, including making his throws."
Yes, that's a part of fielding so it would have to be considered. For example analyzing a shortstop defensively I would need to know if he can 1) glove it and ) make an accurate throw.
Arm strength stems from this but can be measured in an entirely different fashion.
But then why have the categories of hitting power and arm strength in the first place? Are they different categories, or are they just subcategories? How would you have rated Wade Boggs or Tony Gwinn in the hitting category?
Hitting, power, and arm strength are tools. Major league organizations draft players on the tools they presently have and tools they may project down the road. Of course a players rating isn't the only thing that makes up a player, but it's a good reference point on how that player stacks up against his peers.
If I was rating Boggs or Gwynn based on hitting there is no doubt they would be 80's with the way they dominated games. Ralph Rhymes isn't Boggs or Gywnn when it comes to hitting and he's yet to make the switch from aluminum to wood.