I'll agree that it can be beneficial to correct form...as a warm up. one would expect a beginner to be perfecting form before beginning a program no?
I also mentioned 5-6 reps specifically for COMPOUND movements as these are what everyone, beginners to advanced, should be looking to progress in to increase strength and muscle mass in the fastest possible way (hitting your type 2 muscles which have the biggest potential for power and growth) & by this I mean heavy duty barbell exercises: squats, deadlift, bench press, shoulder press etc.. It will take a lot less time and struggle to move up in weight (and thus build more muscle) in the lower rep ranges, not going to failure, than it will to move up in weight doing 12-rep sets to failure & frying your central nervous system every other day. Of course, this wouldn't apply to someone on Vitamin S for example
That's not to say doing higher reps is a bad thing for a natural, as you correctly say it's beneficial to maximise growth by hitting both types of muscle fibre. I absolutely advocate going higher reps on dumbell exercises and accessory movements like curls, extensions, leg press, dumbell chest press etc..heck even going to failure because muscles such as the biceps or traps are smaller and easier to recover sooner from. This way you are eliciting both mechanical and metabolic tension.
I think you will find studies that support both sides of the fence in regards to hypertrophy tbh. Take the following study from last year which took to groups of men who trained in the moderate-intensity, high rep style and the high-intensity low rep style:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4562558/
"In conclusion, the results of this study indicate that high-intensity (3–5 RM), low-volume resistance training program utilizing a long rest interval (3 min) is more advantageous than a moderate intensity, high-volume (10–12 RM) program utilizing a short rest interval (1 min) for stimulating upper body strength gains and muscle hypertrophy in resistance-trained men during an 8-week study..."