For my warrior, 20 defense equals:
0.13% dodge, 0.12% parry, 0.17% block, 0.09% chance to be missed.
Now lets compare that 20 defense to the alternatives in block rating and block value on a mob hitting for 20k, assuming our current stats are 23% block, and 2000 Block value.
Our 20 defense gives us 0.36% avoidance, and 0.17% block. The 0.17% block is the equivilant of 0.0221% damage reduction (2k block value mitigates 13% of a 20k hit through critical block). So we'll add our numbers together and say 20 defense reduces our damage taken by 0.3821%
Now lets look at block rating how much block rating we need to match that damage reduction. We know it takes us 16.39 block rating to gain 1% block. We also know that a blocked attack reduces the damage of that attack by 13%. This means that to gain 1% damage reduction, we need 7.69% block. To gain 0.377% damage reduction we'll need 2.9% block. That is, 47.53 Block rating to get 0.3821% damage reduction
(the equivilant damage reduction of 20 defense).
We have a 23% chance to block, and we need to find out how much block value it would take to equal the average damage taken reduction of 20 defense (0.3821%). 0.3821% damage taken reduction on our boss that hits for 20k means 76.42 damage. We have a talent increasing our block value by 30%, and a 30% chance to block double the normal amount, meaning that we need ~45.2 Block value to match 20 defense.
Now these numbers are for a boss that hits for 20k, which isn't esspecially hard for an Ulduar boss. If you take a boss that hits for 25k, both block rating and block value lose another 25% of their worth. I think block value might become worth while in 3.2 - atleast until you start getting to hard modes, block rating however will still be a pretty terrible stat.