Joe Dupre'
Omono
In low quality tools, the nub may not be in the exact right location, and it just can't do it's intended job. Then, yes it's just there for window dressing. I've been using the same pair of concave cutters for eight years now. They have a tiny, shiny contact point where the nub meets the handle. Unless you sharpen too aggressively or abuse the tool, I would guess the nub and the edge would wear at approx. the same rate and be kind of self-regulating. If you have the need to adjust the edge contact, CAREFUL, slight filing of the handle contact point would be my choice.