There is a reason why the Warrior MTs of most of the worlds top guilds raiding at the cutting edge of current raiding are using mongoose. Mongoose when procced will give about 2% dodge for a period of 10secs. That will be for about 4-5 boss hits. More if we are talking about Algalon which hits 2 times per sec. So basically you have this avoidance buff for 4-5 boss attacks on average.
Now lets look at bladeward. Once the damage effect procced, Bladeward has an ICD of around 45secs and will not proc the +parry effect until the ICD is over. This means that you are effectively parrying 1 attack from the boss every 45 secs. No doubt you get about 3-4% parry from 1 charge of the parry proc, but end of the day the bladeward buff still only allows you to avoid 1 boss attack every 45secs.
TBH i was very excited to see bladeward when it was released in ulduar but in it's current form it is a very lackluster enchant especially when it was supposed to be the "premier" tank enchant of wotlk.
Of course if you really like bladeward, and are adamant that it is superior with whatever reasoning you decide to justify bladeward, go ahead and use it.