(beware, I'm a card carrying kilt inspector)
I'd just like to put this here, and it's actually the appropriate forum for it.
First off, there are no "rules" for wearing a kilt, one could get into the use of the language around kilts ("commando" and "regimental") being slang invented by the English and thus the language of the oppressor culture, but that's immaterial.
I wear underwear with my kilt, and I encourage others to do so.
I do it for three reasons
1) I am not so blessed as to have "male thigh gap" and frankly if it's too hot or too humid I'd rather no deal with the chafing on my thighs.
2) In non burning man situations politeness demands that you not allow an errant gust of wind to expose your cock to those who have not asked to see it.
3) This one if the most important: ASS PRINTS. 8 times out of 10 when wearing a kilt and sitting down your ass or your balls are going to be in contact with the surface you're sitting on. If we encourage naturalists to bring a towel so they don't get their sweat, smears, dingleberries or crabs on our furniture, then we should encourage kilt wearers to do the same.