If you mean that "serious beginner" is a blanket statement than I might see where you're coming from.
So with the kind of difference that you describe between brown and black in NGA than maybe there should be belts in between brown and black. It doesn't really make sense to have going from brown to black to be like going from white directly to black. That's an exaggeration of course.
We've never had a need for any other ranks in there. It fits the progression. By brown belt, students have received the entire core curriculum (only a few bits held back for yudansha to learn) and met the minimum standards for defensive use, so the path from brown to black is about improving ability and learning to teach. It would be safe to compare our ranks to what you're referring to by aligning mainline NGA's purple with the shodan you're thinking of, then keeping each step of promotion aligned (brown = nidan, shodan = sandan, teaching certificate). It's really not such a big jump, since there's no new curriculum introduced in that interim - just a lot of new responsibility accepted. As long as we all know what our own ranks mean, there's no confusion.