I just wanted to point out that in Gazhole's post, every single one of those programs utilizes the optimal hypertrophy intensity zone. Even the 20 rep squats program calls for using your 10RM, and doing breathing squats for the remaining repetitions. Again, this doesn't mean that you can't produce hypertrophy by lifting somewhat heavier or lighter than your 5-12RM, but there are practical limits.
If you sit there doing 1-3RMs for your whole workout, you'll fatigue before you accumulate enough volume to maximally stimulate hypertrophy. Conversely, if you use weights that are too light, it will eventually turn into something aerobic. Running and walking are resistance training in a very literal sense of the phrase, but the resistance isn't enough to stimulate hypertrophy, no matter how much volume you use.
Training age, as stated before, is another important training variable. Very deconditioned individuals, or just average Joe's new to resistance training can get a hypertrophy stimulus from much lighter weights. More seasoned veterans are mostly wasting their time if they go much lighter than 70% 1RM. The minimum threshold necessary to stimulate hypertrophy goes up with training age, but goes down as you become deconditioned.
With that said, Gaz's message of progressive overload should be taken seriously. I don't care if you're lifting in your optimal range if you don't attempt some form of progression. Progression is the most important element in ANY type of exercise program where you want to see performance improvement. You will see better results using your 15RM combined with proper progression than using your 8RM without ever attempting to progress.