Knuckles make horrible weapons for many reasons, including:
hard to put on
hard to remove (injury to hand)
most aren't fitted properly (injury when used, or ineffective at the best)
most aren't trained to punch properly to begin with, let alone how to use knuckles effectively (technique is a bit different depending on your goals with it).
Honestly, a roll of nickels does a better job. When I was too young to carry a weapon but needed something (joys of growing up in a high risk area), I used to use a piece of dowel rod to do the nickel trick...all it does is make your hands more rigid, allowing for more power with less risk of breakage, provided you picked the right size dowel.