I don't know how others are wearing their kilts, but the only time the kilt comes in contact with my skin (the back portion) is when I am sitting down. I can see the arguement about sweat and such on the wool, but honestly the kilt rarely touches skin. If I am wearing my wool kilt, I am likely dressed pretty nice, so why waste that by sitting for long periods of time? If I am wearing the kilt to work, it is likely a PV kilt, so who cares if I need to wash it more often.........it's machine washable

Otherwise the shirt is tucked in and the waistband of the kilt doesn't touch skin. The hang of a traditional kilt doesn't have pleats up against your butt or legs. I prefer regimental 99% of the time, any part of my body that the kilt touches is an area I was taught to keep clean from the toddler years, so I am not sure how underwear is more hygenic. I worry more about the mess on my hands from food or drinks I may have while kilted.

Everyone has their own opinion, that is just mine.