That's akin to never rereading a driver's manuel because you've been driving for years. But the average person who has been driving for years can't pass a written or practical driver's exam. Not untill they re-read the manual. Nor does rereading the manuel meen you doubt your skill as a driver.I understand the value of the progressive path, and I understand teh value of maintaining your basics throughout time, but I think that purposely doing forms slow because of doubt, or unsureness in your self is detracting from being able to actually develop the techniques.
I do think I see what you are getting at, though. You are thinking that that one who practices forms slowly never do any thing but practice forms slowly. You would be right that this is a waist of time, but that doesn't invalidte the practice of doing forms slowly. I doubt any who does forms slowly only does them slow.