Lowering number of shots attempted by big men still isn't full success of defence because other player still can score. And please imagine what can happen in scenario like this. If your the best shotblocker is C, 2nd is PF and subsequently SF, SG and PG and the best inside scorer of opponent is PG, and next SG, SF, PF and C in that order then your SB will do opponent a favor.
So you need to always set the best shotblocker against the best opponent's inside scorer or you should have all player with the same SB, and it's impossible, escpecially if you train SB.
Meanwhile there is no problem with ID because it works as team stat, not as head to head like SB.