[Gwathagor]

I agree that anyone who got wraith-ized with a black stabby wraith knife would not have been a Black Rider-level wraith. They would be a different variety of wraith altogether, as their wraith-ness had no connection any of the Rings, really. Had Frodo been successfully wraith-ized avec wraith knife, he would have become a puny, flimsy, useless, drifting, invisible spirit. The point of the wraith-izing morgul knife was to take away power, not to grant more of it. They were trying to make him as thoroughly weak and helpless as possible.