For me, using the belt sander without the platen is smoother and easier than using the platen and grinding a flat bevel edge. I can vary the curvature of the edge by changing pressure or the part of the belt I use. For a large sweeping full convex spine to edge zero grind, I remove the platen altogether and get a lot of flex in the middle area between the wheels. If I want the edge less curved (less convex) and flatter, I grind near the top wheel with light pressure, causing less flex in the belt.