The importance of windscreen wipers should not be underestimated. I remember my favourite airplane in Microsoft Flight Simulator 2002, it was a special 737 download model which had controls for pretty much everything you could imagine. Even windscreen wipers. They didn't make any difference because the rain back then didn't accumulate on the windscreen but the awesomeness of putting them on was great!
I agree that the automatic wipers in GT5 is an assistance that I would rather want to control myself. It's just a small thing, but measured in awesomepoints it can make a big difference.
In GT6, can we hope for dirt and insects accumulating on the windscreen? Then we could have a reason for turning the wipers on even when it's not raining