In pvp, it is placebo and does nothing against a crit. If shields blocked 100% against crits they would be valuable and provide an option to be used in pvp.
in pvm they help you create a solid tank against 30% of the hits that come your way, 50% if you have certain gear.
Right now we have an issue pre-pvp with build diversity and gear diversity, it is not what was envisioned a year ago with the "thousands of options". However adding shields as a guaranteed block would help create diversity in play styles in both pvm and pvp.
2 WD passives = 25% damage reduction
shield = 32% chance of 4k reduction
gear set + items = 30% reduction against elites
resist = 600+
armor = 4000+
= same amount of defense vs a crit as without the shield.
a 100k hit vs 96k hit is nothing because the next hit is death. the other 2 hits would of landed for 100kdps anyway. at least with 30%-50% block rate (all geared out) you would have 30-50% of negating the critical damage 100%.
this game has become speed + crit + stat, there isn't any diversity and it hasn't been addressed in the QA or the recent small patches.