Well, I always keep the door on my Neptune 7500 closed when it's not in use, and it's never had a problem with mold.
But the 7500 also has a fast last rinse tumble pattern to clean the inner and outer tubs of any mold food. I also use STPP in nearly every load, which helps prevent washer deposits and also gets the clothes and washer very clean. Add to that, at least one hot wash a week.
Moral: no food for mold, mold won't grow. Mold can't grow with just plain water and a plastic or stainless steel surface.
So, in other words, I wouldn't necessarily blame the tub light for the Neptune mold issues. I don't think leaving the door open will eliminate all chance of mold if the washer or detergent is leaving mold food in the washer, and numerous complaints of mold "even though I leave the door open" in various front loaders would seem to bear this out.
By the way, it's not difficult to put the washer on a power switch so that one can cut the power to it when it is not in use. A side benefit for the 6500 and 7500 washers is that there will not be a phantom electrical load, and the circuitry will be protected from power spikes/surges while the switch is off.