Actually, DTrain, if you read Jamie's original post, it seems to be a discussion on knowing the rules and not a thread on the rules themselves. A couple of things on this.

First, those who want to know the traditional way of wearing Scottish attire can easily find out by reading; for example, the fairly inexpensive book So You Want to Wear the Kilt? by Thompson.

Secondly, I think there is some frustration with traditionalists with those who ask about the proper way of wearing Scottish attire, and, not liking the answer, say, "Rules were meant to be broken."

Finally, Matt is correct. Many people associate "traditional" with formal wear. But a man wearing a traditional kilt and t-shirt only, while fishing at the River Spey is being every bit as "traditional" as the man in the PC. As Matt said, it's simply that there are more social conventions when in formal attire. This is true of every culture that has formal and informal wear.