I think Pumpster is claiming that unless the muscles have to do something extraordinary then they won't grow. I remember reading what Arthur Jones wrote in the early 1970s. He discussed which rep actually made a difference. It seemed to make sense that the rep you failed on was the one that benefitted you the most. However, what if you stopped on the rep just before the failed rep? Would that be just as good? I can tell most of the time when I can't do another rep so I stop there. I then continue to do more sets with the maximum resistance. That is the key to hypertrophy as far as I am concerned. Lots of sets with the maximum resistance. No drop sets or forced sets. What you will find is that the reps will drop by about the 3rd maximum set. So I warm up by adding resistance and stop when I can do about 12 to 15 good reps. Then by the 3rd set with this maximum resistance the reps drop to about 8 to 10 which is still good. If you drop to about 5 reps you tend to try to cheat too much to finish the reps. When you start doing that you recruit other muscles to help you finish the set. I superset most exercises so the rest between sets is about 2 to 3 minutes. I find that about 6 maximum sets stimulates growth but I sometimes do a few more sets. I haven't tried this strategy for legs yet but it works great for calves and arms. It remains to be seen what would happen if you kept doing maximum sets for hours on end. I suspect you would grow rapidly, indeed.
Ya the belief, which i agree with (as do some volume trainers, including Schwarzenegger in PI "if you can't go through the pain..") is that the muscle will only adapt if forced to by extraordinary stress. Who can refute this-how will staying well within current abilities force the muscle to adapt? And how does progressive resistance occur while remaining firmly within the status quo?
Secondly, how can you be sure that you've trained to full potential if you consistently stop short of failure? You don't know, and can well be limiting potential growth. The only sure thing is that training intensely to failure is maximizing potential.
Third, to stop short of failure is basically being a pumper, which is one of the disparagements aimed at BBs in general over the years. It's unimpressive as well as unathletic.