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.
Now keep in mind that this only applies to Warriors and Druid tanks. Paladins, on the other hand, can (and probably should) be shielded when they pull. Since they use mana to generate their agro, they essentially already have a full rage bar to work with. The downside to a Paladin, however, is that the mana pool is limited. Even with the ability to gain 10% mana back when healed by an outside source, they are losing the ability to gain mana from over-heals in the next patch, per the PTR notes. This may hinder the sensibility of shielding the paladins when the changes go live, because if they aren't losing any health, then they are losing the potential mana-gain as well.
Many people were requesting good alternatives to the shield, and the answer there is simple. There are spells available in the end-game that give instant heals from taking damage. For example, Prayer of Mending and Earth Shield. Another good option are heals-over-time, or HOT's: Renew for Priests, Rejuvenation and Lifebloom for Druids.
Long story short, its generally not necessary to pre-shield the Warriors and Druid tanks, and it is a good idea to pre-shield a paladin (at least for now.) However, as one of the forum posters said, "there's no one rule for any situation."