Pre-shielding when you know big damage is on the way for select targets is fine. Trying to keep shields up on the entire raid is a strategy that we think A) is too good, and B) isn't fun. I think a lot of the players who think it is fun feel that way because it's overpowered, not because it's a really engaging activity. When your decision making consists of cycling through targets and casting PW:S on any target without Weakened Soul, then you aren't really responding to the encounter -- you're acting like a glorified macro. Now maybe some of you will argue that you take a lot more into consideration before executing a pre-heal, but the fact is that Disc priests can be unbelievably effective without doing so.
...An Icecrown parse for a Disc priest might have PW: Shield (including its glyph) and Divine Aegis at 80% of healing done. Imagine that is more like 50 or 60% of healing done. That's still a lot of power for shields. The remainder would be filled in by spells like Penance, PoM, Heal and Greater Heal.
Now that I have some numbers I can complain about broadsides like this. Suppose you are trying to keep a tank alive and conserve mana, but that just spamming Heal is not enough.
If I am trying to be mana efficient with heals, then I am definitely going to cast Power Word: Shield about every 15 seconds. If I get a Rapture from each shield tick then shield is about 26 healing per mana, while Heal, my next best spell, is 16.2 (more on the efficiency of Heal later) in Tier 1 gear. At final tier shield is returning mana each cast. Renew is, next to Heal and ignoring Rapture, by far my most mana efficient heal and my best healing per cast time, so I would presumably keep that up on the tank if spamming Heal is not enough. I could fill in any additional healing with Penance. Lets assume that's enough and I'm not resorting to Flash Heal to further up my healing per second.
In this scenario tank healing would make my shields about 18% of my total healing (counting both Power Word: Shield, it's glyph heal, and Divine Aegis). Obviously that is making not a lot of us of shields, just one on the tank every 15 seconds. But with a spell as expensive as shield (the first one is free, but each additional one costs the mana equivalent of about 3 Heals), how often can I realistically be casting it? To get my shielding up to 50% I would need to go up to 5 shields per 15 seconds, filling the rest in with a single renew, a single penance and 2 heals.
That would mean spending about 1760 mana per second instead of 1024 mana per second. It would also increase my Raptures, probably from about 1 every 17 seconds to 1 every 14 seconds. So the after rapture expenditures would be 1581 vs. 877. Replenishment is 1k mana per five seconds, so that reduces it to 1381 vs. 677. If we can manage a good 500 mp5 in combat then that would be 1281 vs 577. We get a shadow fiend that gives us half our mana back, so we have about 150k mana to work with.
In other words, if we get about 18% of our "heals" from shield then we can heal for nearly 4 minutes without a break. If we get 50% then we can heal for 2 minutes without a break. The former sounds pretty sustainable, the latter not so much.
But what is really troubling about that particular blue comment is that it is very disconnected with reality. I went and checked some logs. My shields are about 30% of my healing (though I cast renew when I am discipline, which most priests seem to think is crazy). A lot of discipline priests run around 50%. In heroic Lich King, which is pretty much the shield spammiest fight ever conceived I found numbers as high as 70%. Regardless of what people would have you believe on the forums, Discipline priests (at least ones who get anything done) don't just stand there and spam shield and right now it is our both our most mana efficient and our highest hps single target heal even if you ignore Rapture. It would stand to reason that when it is significantly less efficient than penance and much, much less healing per second than renew we will cast it less, not more.
The reason this is very important is that we have a mastery bonus, and it is balanced around how much of our healing comes from shields. If we get 50% of our healing from shields then our 20% more absorbs mastery bonus is nearly 10% more healing, and any bonus we get to that from mastery rating will be about .5% more healing per 1% mastery bonus.
If we get 25% of our healing from shields (probably a more realistic number) then our mastery bonus is half as good as that. It is very important that the developers have a good idea of how much healing is actually going to come from shields.
I think we all agree that is it less than now. It would be nice if we could agree on the reality of what it is now so that new numbers could be based on that.