Each of us has a finite number of androgen receptors. Of course they change with time and steroid use and get upregulated etc etc etc, but at some point, every AR will be met with a steroid and you will essentially run out of parking spaces for the steroids. At this point, the extra steroids will still be beneficial because they can still work on non-AR mediated pathways to create things like cortisol reduction, increased red blood cell count, and other effects.
Here is my point:
If we only have a limited amount of androgen receptors, shouldn't we make sure that only the strongest and most effective steroid interacts with our precious few, limited androgen receptors? Isn't taking a drug like primo or anavar essentially a waste of androgen receptors? Even boldenone? You could get better results with only testosteorne. Boldenone is going to exert similar effects to testosterone - only it will do a poorer, weaker job of it. In other words, boldenone takes up spaces better used by testosterone.
The only reason to take weaker drugs is if you are looking for particular effects: no water retention, lots of water retention, safety, etc.
Still, if someone simply wants to bulk and get huge, isn't it misguided to take 500mg of testosterone and 600mg of boldenone? That is a total of 1,100mg of steroids, but won't produce gains as good as 1,000mg of testosterone alone.
Conclusion:
If a steroid user is not concerned with taking a mild cycle, and simply wants to get as huge as possible on a hardcore bulking cycle, then it makes sense to only take the drug most effective at the androgen receptor, and stack it with another drug that won't interfere with the spaces at the AR intended for testosterone. For example, Anadrol binds poorly at the AR, and will leave testosterone and the AR alone to do their work, while Anadrol goes elsewhere (mostly non-AR mediated pathways) to do its work.
If I only have a limited number of androgen receptors, I want to make the most of each one, and only use the strongest, most effective drugs, rather than waste AR space on poorly binding, less effective, expensive drugs.