I'd still expect a ductile failure, not a rupture, from a single overload even with an ugly drill hole.

Without pre-bend Wouter is correct. In that case there would be negligible tensile stress in the pole. The upward force applied by the spinn should be counteracted by the lines that extend from the bows to the point where the tack of the spin is attached to the pole. Because the pole extends in front of the bows, those lines also exert an inward, compression force on the pole. Resisting that compressive force is the pole's main purpose. Essentially the system is a truss. It would be a pure truss if you had a hinge in the pole at the bridle connection.

With pre-bend the pole is also a cantilever beam between the bridle and the spin tack. Fatigue damage accumulates every time you rig the boat, and when the spin is unloaded (every dowse, gybe etc.).