I've noticed with my bigman that even the ones with lower IS< or even as the one whom didn't ave low IS, but went against bigmen with better ID, if the bigman they were against didn't have decent SB, even with low IS, as long as my bigman had rebounding at par or better to the other guy, they would rebound misses and tip them back in.
I agree with you that IS is overrated. But for the U21 player am training up, I am following the advice of the people who want him trained with the IS and all the shebang etc.