Paul,
I think the point here is that IF one is cured of some disease or affliction and it is verified in some way, saying well that can't be true because there is no record of faith healers causing new arms to grow, in my view, in no way invalidates that there is no valid faith healing. The question of whether 'faith healers' have even prayed for arm amputees growing new arms after prayer, i am not aware of. Why God would give faith to an individual for one thing and not another IS a valid question for which i personally have no answer. But to me the argument does not invalidate 'faith healing'. Of course if one thinks it does, to me that is fine as i have no quarrel with those who choose to believe differently. I would just disagree.
Just my own thoughts,
Joseph