Rubber is rubber, and does not stick to ice, whether it's knobbies or slicks. If you've ever been walking along and suddenly slipped on ice and ended up on your butt you know what I'm talking about. Knobbies will help in deeper snow, but if there's ice underneath you need studs.

Yes, those are metal studs. Steel, with hardended carbide pins, to be exact. They will bite into ice but will not be damaged by bare pavement - they will be pushed back into the tire and pop out again, ready for the next patch of ice.