Without the cutout relay, they would discharge the battery they just charged, burning themselves out in the process. Alternators use the internal diodes to accomplish the same thing. You will need a regulator for generators anyway since the cutout relay is necessary. This is due to to generators are basically electric motors, driven by the engine, to charge the battery. Without the cutout relay, they would discharge the battery they just charged, burning themselves out in the process.
Alternators use the internal diodes to accomplish the same thing.