Rationale comes from the theories/practice that supports my prejudices, if you will.
Sort of very simply...
http://www.t-nation.com/free_online_article/most_recent/too_much_muscle_the_glenn_pendlay_secret
I can go and dig out more from an academic perspective you want, but I know that's not why you're asking.
The premise being that compound exercises use more muscles, which elicits a stronger hormone response from the body. Going bigger (heavier) on those compound exercises, therefore, adds another factor into this equation.
So, going for lighter loads (I'm assuming the OP is, based on the fact that he can do lots of chest work after his bench) across multiple exercises will not stimulate the same effect, because the muscle stimulus will not be there to the same degree, and only smaller/fewer muscles will be worked.
Shot full of gear, and it's a different question.
My perspective is that - after a proper set of compounds - the individual should not really be able to work other, similar movements: the fact that illuz can do variations of flys after his bench suggests he's not getting all he can from the compound movement. Yes - do a 'finisher' to really exhaust a specific muscle in isolation (hence why those exercises exist, IMHO), but this should be thoroughly painful and not something for which people would do multiple variations.
I am well aware that time under tension is critical for mass, which is why cable work is important to BBers, and would never suggest they drop time under tension as a basic principle. What I am saying is that this can (IMHO) be achieved in a more effective way - given his training background - than currently.
@ illuz: do bear in mind that I am a nothing in the grand scheme of things on these forums: steedie has helped prep people on here for photoshoots as well as competed as a bodybuilder himself. I have done none of these things.