"This will do NOTHING to extend clutch life. In the days before synchronized transmissions, this procedure was used to prevent gear-grinding. Most of the wear on your clutch occurs when you are starting off from a dead stop. What you are doing will accelerate the wear on a critical clutch component, the throw-out bearing… "
Sorry C-man, but I didn’t say it would extend clutch life, I said it would minimize transmission wear. It will do just that, since it effectively minimizes the job the synchronizer blocking rings need to do. It was, as you say, done to drive transmissions that weren’t synchronized at all.
As for prematurely wearing out the throwout bearing, well, done correctly double-clutching spends very little time with the bearing loaded. I could make an argument that the rev-matching eliminates enough clutch slippage that the heat NOT generated more than makes up for any mechanical wear on the bearing. But I guess it’s a bit of a trade-off, a little more bearing wear or a little more transmission wear. Six of one / half a dozen of the other.