Casual I agree but not with the example you gave. People, steroids do make up for bad training routines. Those people that are huge doing 25 sets for their chest? 95% of natural trainees couldn't get away with that. But Mentzer is not a good example. Heavy duty may not be the absolute fastest way to gain muscle, but it definitately works. Most natural trainees would gain on this program as opposed to a volume routine. Heavy Duty isn't very dissimilar from HIT (hell that's where HIT came from) and I've seen HIT make a lot of regular people muscular.
Let me pose this question to everyone:
How many large, natural trainees with average genetics do you see doing a traditional volume 4-5 day split? I have yet to see one. All of the huge natural guys I've seen or talk to do some variant of HIT, DC, or a powerlifting type routine. Simply because these programs allow one to consistantly progress and achieve great levels of strength, and eventually great levels of size along with it. I'm sure HST is the same way but it hasn't been around for a long time yet.