"some leave the battery on, and hence cooling fan, even after shutting engine down until temp drops and fan cuts out, can't see any reason for it myself but...........?"
Yes, another conundrum! I would think that leaving the cooling fan going after shutting the engine down will only cool the oil in the cooler, & subsequently the reduced temperature oil from the oil cooler drains back into the resovoir, reducing the temp at the switch & shutting the fan off. I don't think this will help the rear bearing at all, but if you have a good battery I can't see that it would hurt anything either.