The forums are abuzz with the debate on when to use the ability "Power Word: Shield" on the tank in your group. The original poster on the forum asked why pre-shielding the tank was looked down upon, and the response was abundant!
Warriors and feral Druid tanks need to generate rage to use their abilities to generate the rage required to hold a mob's agro so that it doesn't run rampant in the group, slaughtering the squishies. They generate rage by being hit by a mob and also by hitting the mob as well. In order to be a raid/heroic tank, you have to have your mitigation and avoidance up to a point that you're almost always dodging, parrying, or blocking, in order to not die from a single hit, but in turn that will decrease the amount of rage you can generate. "Power Word: Shield" only decreases the rage generated, while not protecting the tank from that much damage.
On boss mobs, it gets even more tricky. In order to successfully hold (most) bosses, you have to front-load your agro and maintain that agro lead throughout the fight. This means that they need an abundance of rage up-front in order to establish that agro, and they also need time to build up that buffer to keep them ahead. Most tanks only need a few seconds to accomplish that, but the shield just prolongs that time. This also hinders the raid on fights where time is a factor. For example, the old Kazzak enraged after 3 minutes of fighting (he probably still does), and every second of dps counted.