There's a good discussion ongoing on the World of Warcraft Forums regarding the Holy Spell Power Word: Shield. Is it practical to pre-shield the tank before going into a fray?

The community is a bit split in two on the subject as some players think that it's useless for some tanks while others think that it has some use. Basically, Power Word: Shield (PW:S) is a defense buff that lets the target absorb a set amount of damage. Warrior and feral shaman tanks need rage to pull and keep mobs concentrated on them (for more aggro). So if the tank isn't getting damage from the pre-shielding, he doesn't get to generate rage either. Paladin tanks however will need to be pre-shielded (at times) since they don't have rage and will need the extra protection.

Things may change after the World of Warcraft patch 2.1.0 goes live as the Paladin rate for regenerating mana from outside heals increases to 20% from 10%. So it would be more practical for Paladins to take damage to get the subsequent heals and mana. Blizzard poster Nethaera suggests that players ask the tanks whether they want to be pre-shielded or not.

So if we're talking about practicality, Power Word: Shield may not be one of the best things to do to protect your tank. Dan Crislip from WoW Insider gave a number of things that can be used as better alternatives: Prayer of Mending, Earth Shield, HOT's (Heals Over Time): Renew for Priests, Rejuvenation and Lifebloom for Druids.