If you improve sharpness at one focus point at the sacrifice of others, what good is it? When you're shooting action using a zoom, then why limit yourself to one focal length by having to avoid others because they won't be as sharp after doing a M.A.? With a zoom it may help in one focal length but it's not an all encompasing solution that's going to optimize focus performancce throughout the range. That's all I'm saying.
You're assuming I'm giving up something on one end, I'm not really. I'm perfectly happy with how my MFAing has turned out, but that's not to say others have not. It is probably a crap shoot to a certain extent. Body and lens combo maybe.