I would imagine that completeness follows the all-or-none principle. You either have it or you don't. Since Roxas only has a fraction of a heart, he isn't a complete being. He's still non-existent.
After all, Ven couldn't function with only a piece of his heart. As for why Roxas can, it's because the heart is not necessary in the first place- a Nobody normally lives without a heart, so giving one a fractured heart would only be supplemental (but would not be enough for them to be considered "whole").
Roxas fits the broader definition of a Nobody, being that which is left behind when the heart leaves its vessel. This time, the left overs happen to include a fractured heart. However, naturally he's comprised of more than the normal Nobody, hence why he's special. So the answer to whether or not Roxas is a "true" Nobody is dependent on what your conception of a "true" Nobody is.
Eloquently spoken, and absolutely correct. Looks like someone did their homework on this one.