I think the pump is important, but not the be all end all. I believe you need to train for strength first and foremost, and "finish off" if you will, the muscles trained with a pump.
Keep in mind when I say training for "strength" I don't mean like a powerlifter 1 rep max kind of strength. I mean strength within your rep ranges.
For example, CG is using lighter weight and cranking out reps. IMHO, if he keeps using the same weight over time he will stagnate. So if he's doing 20+ reps per set with x weight now, then in a year he should be doing the same reps with x+y weight. Where y is really anything, as long as it is MORE than x was by itself.
This is why I really like to train with heavy weight FIRST in my workout and do descending pyramids. I hit the muscle hard and heavy and then lower the weight and get more sets and more volume in.
Volume for me is variable. I train until I get the pump and feel super tight and light weights are heavy. If I ever start to LOSE it, I quit right then and there. That is your sign of overtraining, when you start to lose your pump.