I think Rockerduck and Glomgold both play very important roles in contrast to McDuck: Rockerduck was born into money and didn't earn a single cent of his fortune, while Glomgold earned his money but did so by lying and cheating. Both are in opposition to Scrooge, who earned every cent and did it fair and square.
I agree, though, that as a character, Rockerduck is more interesting.