I'm sure originally blocking was just supposed to be additional mitigation, and as said the crushing blows thing was a side effect. It just happened to work out in a way that they were able to manipulate into a raid tanking mechanic; they've left it in for so long now that it would be class breaking to remove. Its all just speculation of course, but they seem to be using it in a positive way now.
Looking at T5 and especially T6 you should be forgetting this 85% number, aim for closer to 100%. I'm betting it is very possible to hit full crushing blow immunity without shield block in a full set of T5+ with approriate jewelry and trinkets. Even just looking at warriors in 2-3 pieces with a styleens they are looking at 33%+ shield block, that just leaves 67% to be covered in dodge/parry/miss, would be fun to play around with a crush immune tank!
Just for kicks:
http://ctprofiles.net/5331728
34.37% block
23.04% dodge
16.46% parry
+509 Defense which I think is 5.6% + .04 * 159 = 11.96% miss?
So right there you've got 86% before Grace of Air, Mongoose, Scorpid Sting, Insect Swarm, Blessing of Kings, or anything else that further increases avoidance.... You can bust 100% easily. This is also rounded gear, over 13,000 hps and over 14,000 armor makes me think you can actually tank raidbosses like this and just sit there without using shield block.... You could hit 100% unbuffed if you skimp your hps but it isn't practical. Someone should do this with Tier 6 and see what they can get.
EDIT: The combined T6 difference in this particular set is + 43 defense rating (17 Defense, .68% block, dodge, parry, miss), +22 block rating (2.75% block), +52 parry rating (2.3% on new formula), -6 agility (-.2% dodge) and +1 dodge rating (+.05% dodge?) So Tier 6 would make the total avoidance 93.32 on this set I believe.