I speak with the weight of virtually no practicaly experience, BUT, I agree with both sides here. I know that you require a certain amount of mass for the motor to keep roating at an idle under minimum fuel application. HOWEVER, the less mass you have the faster the motor will rev.
I used to run lightened rods in the VW and porsche motor s that we put in sand rails. We never had a problem with breakage no matter how light we went, but the mild cammed motors wouldn't idle well with the light rods. The heavy cammed motors idled higher though, and didn't have that problem, and did produce substantially more HP, but less torque.
I think the real direction of your question might be, " how much lighter can I make the rods in my _______, and still idle well at ________, have good general driveability, and not come apart at __________ rpm. "
yet another useless o2