The lead-weighted gloves they're talking about are more like saps than a traditional knuckle-duster. They only really add weight (which is nothing to sneeze at.)
Metal knuckles and the like add weight AND focus a strike on small or sharp points. That's what brass knuckles do.
The lightweight plastic and carbon models (I haven't seen those, but I'm willing to be somebody makes them and Skunkabilly can probably tell you where to get them) would still focus the force on a smaller area, but would not add weight.
They're not terribly practical for defensive use, but I suppose if you need to sneak up on somebody and knock him out from behind, they'd be OK. For defensive use, the question is, do you go around wearing the things, or do you hope you can get one out of your pocket and onto your hand when you're attacked? Better just to punch him.