The mast functions in part as column and in part as a beam. As a column, the compression (gravity-like) loads are limited by compression buckling. To protect against that buckling, the "column" must be restrained. The stays provide that restraint. The choice of a thicker walled or bulkier (and heavier) mast vs. one that is restrained by a more complicated multi spreader rig is what is driving the decisions. And yes, a smaller section on the mast makes it easier to bend.

When you intentionally impose a bend on the mast, you also increase the compression (gravity-like) load. That makes it otherwise more likely to buckle too. To restrain that intentional bending loading, the restraint analysis leads to multiple spreaders.