I've worked with old timers that drove trucks forever that double clutched until they've retired. Don't think it means inexperienced. I'd prolly be double clutching still if a mechanic didn't throw a fit years ago when he had to adjust my clutch twice in the same month.
I think there'd be a problem somewhere else. Sometimes the shop doesn't know as much as they want us to believe.
Glad to learn something new. I don't see how you can shift faster, but I'll take your word for it either way. Heaviest I've been is about 95k, and probably hitting 4% grades when I was...although hell in the mud, I doubt it was near as heavy as y'all are talking.
Even at 95,000lbs. I would probably start looking at using the clutch to shift. Over 100,000lbs. and I, at least, will be using the clutch getting going on an incline.