To me, this always just seemed like something auto-xers made up
I can see how auto-x helps hone your reflexes, car control, etc. but those are all skills that can be learned on the road course as well. If you ask me, road racers make good road racers.
That being said, there are a lot of great drivers who started out in auto-x and moved on to become road racing legends (Randy Pobst comes to mind). But there always seems to be an ongoing feud between auto-xers and road racers, and this statement always gets used. I just never got into auto-x because it's a lot of standing around and very little time in the car. It is good for the budget though.