Wait, this is an original D&D rule ???
Then I would have to argue that a potion recipe actually demanding fingers of a sentient race is a bloody obviously outrageously stupid idea.
But as anyone familiar with the subject knows, D&D is certainly not above having outrageoulsy stupid ideas at all.
I guess the giants could just rid themselves of their ten fingers and afterwards just cast a sufficient cure spell to regrow the lost bits, but still.