Sure, those enhanced bodybuilders can get bigger with high-rep sets. Here's why that's not optimal for you.
Protein Synthesis: Juicers vs. Naturals
Steroid-using lifters can gain a lot of muscle from doing plenty of work with lighter reps – things like high-rep work, drop sets, etc. That's because they don't need to trigger protein synthesis with the workout itself. Building muscle is all about protein synthesis. The higher the rate of protein synthesis and the longer the rate of protein synthesis, the more muscle is produced.
Drug users have an enhanced level of protein synthesis 24/7. But a natural lifter's normal state is homeostasis: protein synthesis and protein breakdown are pretty much equal. He needs to stimulate protein synthesis with the workout itself.
When an enhanced bodybuilder does very high rep-work, he'll drive more blood into the muscles, and if that blood is loaded with amino acids, they're then supplying their muscles (that already have an enhanced level of protein synthesis) with the nutrients needed to take advantage of their high rate of protein synthesis.
As such, steroid users don't need fairly heavy work as much as natural lifters do. In fact, since many steroids tend to make the muscles proportionally stronger than the tendons themselves, focusing too much on heavy lifting might lead to devastating injuries.
So Never Do High-Rep Sets?
I'm not saying that light pump training doesn't work for natural lifters. Hypertrophy can be stimulated with light weights if you go to failure.
The problem is what happens in the body when very high reps are combined with light weights. Sure, you'll eventually get maximum muscle-fiber recruitment as you build fatigue during the set, however, if it takes you 15 or 20 reps to "get there" instead of 6 to 8, then you're spending twice as much glycogen to achieve the same effect.
Remember, the more energy (glycogen) you need to mobilize, the more cortisol release you'll get! That's why super-high rep work is not ideal for natural lifters, especially if you do a lot of sets.
Generally speaking, most sets should be in the neighborhood of 8 to 10 reps, with higher-rep sets reserved for warm-up or preparation sets. However, if a natural guy is going to do lighter pump work, he should make sure to consume a carb/protein blend pre-workout and during the workout. (Plazma™ is the top choice here.) This makes glucose readily available for fuel so he won't need to mobilize as much stored glycogen and he won't release excess cortisol.