While i'm aware that you get more damage reduction pr. point of avoidance than pr. point of block, i think it's important to look at how that damage is reduced.
Blocking alongside crit. blocking is basically an extra level of mitigation. We lost quite a bit of armor, but gained a flat 30% damage reduction when blocking, a tradeoff in our favour especially when factoring in the 100% block chance from Shield Block. Dodging is still a 100% damage avoidance on a single attack whereas parry is a 100% damage avoidance over 2 attacks, effectively lowering the incomming damage of 2 attacks by 50% each.
Having a relatively high parry chance combined with a well buffed mastery will ease out the incomming damage more than the "pure" avoidance will. And that combined with the change to the pace at which healers can throw heals, makes the most sense to me, even if it does come at a higher statpoint cost.
On top of this comes the improved rage flow as you mentioned, if you spec shield specialization. And a stable rage flow is key to tanking successfully at the moment. Avoiding attacks simply cuts down on rage too much.
All in all i'd say that allthough it might be abit more expensive to get mastery up, it's worth it.
Personally, i reforged the dodge stat on all my tank items into mastery and kept the parry rating at 22% (for a nice secure uptime on Hold the Line). That leaves me on aproximately 22% parry, 17% dodge and 41% block and several healers keep telling me that i'm so easy to keep up compared to other tanks. My health used to fluctuate rapidly when tanking, but now it's alot more smooth.
The rage flow is stable enough to allow a near-spam of Heroic Strike/Cleave while maintaining debuffs.
Atleast for me, it works out.