Block Rating really isn’t a great stat for Prot Pally's (or warriors for that matter). For the discussion I’m going to assume you know about the attack table http://www.wowwiki.com/Attack_table if you don’t.
Basically the first thing to fall off the chart after we reach 102.4% combined avoidance and block rating (this is were hit gets pushed off the table and we become unhittable) is block rating. So if you think about a normal set we get 30% from holy shield already so at an averaged avoidance level of say 55% we are already sitting at 85% towards the unhittable cap. Even in a set devaluing SBR you will generally sit at around 15% so that give us a total of 100% already. So basically any SBR will be wasted because it will fall of the end of the table.
I haven't even mentioned Redoubt
TL|DR it falls off the attack table to quickly to really care about.