they can feed electrical loads, as the diesel in diesel loco is used to drive alternator, so you readily have 750V AC output available at hand, but since Diesel loco don't have an almost limitless power supply (unlike electric loco which can tap power from grid whose capacity is far, far more than a single loco) doing so means that lesser power is available for traction motors, so in essence having diesel locos on HOG derates it to a lower value. In electric loco the problem is remedied by having larger traction transformer, which allows you to feed electric loads in the train without compromising on power available for TM's.