I remember when I first rolled my priest my friend had told me that at lvl 6 (back in wrath) I would get a shield. That shield would soon become my best friend and still is one of my favorite things about my priest (even though I'm holy!). After the recent buff and subsequent nerf to Power Word: Shield with 4.0.6 I started thinking about the mechanics behind shields, how they work, and why they are so effective. So after some thinking here are some of the reasons I find shields so effective:
One of the biggest reasons people cited shields as being to be nerfed is there abillity to trivialize certain mechanics in encounters. Being able to shield half a raid to minimalize the healing needed is a very powerful ability. Good discipline priests are very effective at being proactive and anticipating incoming damage and minimalizing that damages affect.
Strong Burst Thoroughput
Power Word: Shield is one of the most effictive burst throughouput spells in the game. Being able to use an instnat cast to mitigate 25k or so damage (which is saving you from healing that) is extremely powerful for an instant cast! Along with the ability to layer other heals on top and you have some extremely powerful burst "healing" (healing+mitigation). That is part of why disc was long one of the best pvp healers, a shield+penance is a very powerful combo capable of saving teamates in tight situations.
Shields increase effective health
Bear with me on this one since I am no math wiz and I am going to be using very general napkiny math. I'm more talking about the concept then the exact affect. Lets say your tank has 150k health, your disc priest throws on a shield that mitigates 20k damage, it now takes 170k damage to kill your tank thusly his effective health has been increased. This effect is even more apparent when damage reduction abillities are used.
For example lets say you use pain suppression for a 40% damage reduction and throw on that same 20k shield. Due to 40% less incoming damage that shield is essentially absorbing 28k damage making the effictive health of the tank 178K.
*NOTE* in this last example the math may be off, I think damage reduction effects stack additively not multiplicativy (aka if you have a 30% and a 40% its not 70% reduciton) and I believe they stack mutliplicatevly with shields
For the last example lets say that same tank blocks an attack while pain suppressions on him which is the first attack on a fresh shield. While the dr's may not stack multiplicately for a 70% reduction the shield does stack multiplicately with them. The 30% from the block essentially makes the shield absorb 26k, then with Pain Suppression mitigating 40% of the already 30% mitigated hit, the shield absorbs 36,400. This would make the effictive health of the tank 186,400!
So while I may be a holy priest now, Power Word: Shield still holds a special place in my heart. Don't forget though these advantages still extend to other absorbs such as holy pally mastery, and Divine Aeigis although to an obviously much smaller extent. I hope that some of this might shed new light on how powerful absorbs are.
Feel free to comment or email me about any mistakes I made or any questions you have