I've always heard, and ascribed to the theory, that a Kirkin' was a recent American tradition (and perhaps Canadian). There is only one I know of in Connecticut each year around Robbie Burns day. The smaller one day highland events don't sponsor any that I know of in New England. The New Hampshire Highland Games which is our biggest event in area (3 days 40,000 people) has a well attended church service on Sunday morning at which the clans are represented. The blessing of the tartan extends more to the members of the clans, families, and those in attendance than to the cloth itself. We also read the "Flowers of the Forest" list of those clan members deceased that year and pray for them as well.
Why do we do a Kirkin' of the Tartan? Because it's a nice thing to do.