One thing that would limit its use, you couldn't grow so small that you couldn't hold a metalmind, and same for growing so large. the metal minds would remain the same size. so say, with bolts through the arms, you'd grow larger around them, and they would fall out...
I imagine it, as simply easier to injure.
Another idea, storing pain? useless when you draw on it, but isefull to store is the problem, when I feel it should be the other way around somehow.