Matt Newsome sums it up best in his article on The Introduction to Tartan on The Scottish Tartan Museum's Website. I would think that the non-Scottish X Marks members who pick a clan tartan that they like, educate themselves on it. I am more confident wearing a clan tartan of my choosing than a country or district or generic tartan, not to say that I don't like them. It is emulating a country and it's heritage. Just like the purists who wouldn't wear a tartan they are not associated with, I won't wear a tartan that is not associated with a clan.