I have one of those blue woolly pullovers, but I'm not an instructor, just a 400 hour PPL. No stripes though.
I wear it because it's the warmest bloody sweater in my closet. And I live in Canada, and rather have a weakness for staying warm in winter. In fact it's so warm, I can only really wear it in the coldest weather.
Let's face it, flying a 115 knot Beechcraft is nothing pretentious...
Mike