Not many successful trainees/bodybuilders use Heavy Duty these days...........if any !!
The main reason Mentzer advocated it is because he wanted fame first of all,and he built the vast majority of his muscle mass using high volume training when he was younger..........add in lots of roids and decreased training time in the gym,and he simply maintained what he had already built earlier in his training life usinf HD.
As stated by others earlier in the thread,most guys who do use HD,usually do plenty of warmup sets,building up to their "one set to failure" which actually amounts to pyramid training.