Also, PERSONALLY, I prefer slash pockets to cargo, on a kilt.
If trying to make a kilt be functional but look as traditional as possible, I absolutely agree.

But one thing about slash pockets is that anything you put in there will be lumpy. We're used to seeing it with trousers, where a billfold or set of keys or cell phone makes a lump in the pocket area. But on a kilt it would look just weird. And would totally mess up the clean lines of a kilt, IMHO. So while slash pockets would be somewhat useful, I think they have pretty big limitations on what you can actually use them for. If carrying more bulky items that will make a lump, an external pocket seems to be the lesser of the two evils. It won't mess up the fit or shape of the kilt itself.