First of all, are you at 102.4 with or without holy shield in place. If its with then its irrevalent for tanking adds on heroic anub since the adds will chew through the charges before the cooldown is up. The reason the other people have so much block % on their block sets is that it has a much better conversion ratio (rating to %) than dodge/parry and thus is easier to stack to get Passively unhittable. Above 102.4% passive unhittability, block value >> block rating as you're blocking more / hit.
If not specifically gearing for tanking anub adds, then holy shield can be usually be added to the unhittable % and block value then is more valuable than block rating as you're already avoiding/blocking every hit.