What if the thrower is invisible, and you have no idea you are being targetted?
What if you think you are having a fair fight, but when your opponent is low on health he insta-kills you with harming potions?
What if you are fighting with someone else, when you and your opponent die of harming potions because someone snuck up on/rushed towards both of you?
Harming potions are far too OP compared to any other method of combat.