I spend a lot of time on the WoW Priest forums, trying to give advice to new priests, and just all around goofing off with other priests. The forums get a pretty bad rap because of a few bad apples, but you can meet some really nice folks over there.
There was an interesting post from a Discipline priest, wondering if Disc Priests would ever be capable of providing the kind of throughput that Pallies and Druids can put out. She ran quite a bit of math, and did remember to include the value of shields in her formluae, but the end result was that she felt that Disc Priests were just missing something to give us a bit of Ooomph in the throughput department. What concerns me is that she wasn't necessarily concerned about her throughput so much as she was concerned about the fact that other healers were "destroying" her.
I don't mean to pick on the Priest who made this post - it's simply an example of this strange phenomenon I see all over the place - of healers wanting to compete with one another. I'm not positive where it comes from, but I do have a couple of theories:
Competition for Raid Slots
Former DPS switching to Healing roles
Now, I can't comment on the PvP survivability and healing abilities - I've said it before, I'll say it again - I'm no good with PvP period. But I will consider the other three potential causes.
Competition for Raid Slots
The competition for raid slots is somewhat viable. But this also depends on the kind of raid mindset you are trying to get into. Harcore raiding guilds look at raid makeup for the buffs and integration each class will provide - but after that, what it really comes down to is skill. I've seen more raiding guilds looking for Priests than any other class, I must confess. For the more casual raids, it seems like there's always a welcome spot for a Priest, if there are any healer spots left (and there usually are!). Holy Priests have a versatility to them that is helpful to any raid. I won't deny that it is trickier with a Discipline Priest - but this is due more lack of knowledge about the spec than anything. A lot of people think that more than one Disc Priest in a raid is a waste - and in some ways that is true in regards to raid buffs, but Disc Priests can handle different assignments, if organized properly. Leaders may see them so low on the output meters that they think they are slacking, when we all know this is not necessarily the case. But even leaders who are aware of that particular issue make the mistake of thinking that if there are Pallies in the raid, we don't need a Discipline priest.
I've been guilty of it myself. And then I started playing around with the different uses and assignments for a Disc Priest in a raid, and I've been happily surprised. Strong single target healing does not equate to Main Tank healing only. There are so many bombs and gimmicks in raids these days, that you almost always have to have someone designated for the Slag Pot or Kologarn fist heals. Disc Priests are great for this.
Sure, you need to have enough reflex and skill and knowledge of your spells to do this well - but the fact of the matter is that more than any other healing class, Disc Priests have strong tools to react to random damage.
Oh those damn meters. Many a Disc priest has wept at the damage to Discipline reputation given by those meters. Meters can be very helpful tools, and they shouldn't be abandoned. But comparing a Disc priest to a Druid or a Pally healer is like comparing apples to oranges. If you want to use the meter as an effective tool, here are some tips.
Know the class you are evaluating with a meter. Get something like Recount Guessed Absorbs for evaluating your Disc Priests. Druid overheals don't show up on a meter, because they stop ticking when the target is at full health - but the mana was spent, and therefore possibly wasted. This is why Druids have high output, and low overheals. Pallies have very high output, and lots of overhealing.
If you are going to compare healers, compare them against another healer of the same class/spec. Comparing a Holy Priest against a Disc Priest will not be accurate, nor will comparing a Disc Priest against a Pally.
Do not just check healing output. Check Overheals, Absorbs, HPS, and all the healing numbers. Consider if you asked a healer to switch to DPS. Consider how long a healer was dead or CCed in a particular fight.
You also need to consider healing assignments. Someone assigned to a main tank is likely to be spamming heals, and is therefore expected to have high healing output, and high overheal amount.
Remember that if a healer knows you are using simply a healing meter to evaluate them, they can just sit there and spam heals on one person, and not be helping the raid out at all.
There are certain addons, like Achelon (I think) that can tell you about the last few moments before a player died. Consider time between heals. If you have a Pally that is solely designated as Main Tank healer, and the MT goes down, check the combatlog - did your tank go down because they went a full ten seconds without any healing? This is something to use for evaluation - it's not uncommon for people to get bored with their healing assignments, and start healing others, but it can be devastating if they aren't focusing their attention.
DPS switching to Healing Roles
Don't get me wrong, I'm not saying it's bad for a DPS to switch over to healing. But healing is very different - you are looking at different things and considering different situations. You are reacting to events and mechanics moreso than even a DPS role does.
But the biggest problem I see is that it is a lot more common for players who are accustomed to DPS roles to become obsessed with that meter, and competeing with one another. If you're DPS, you want to have the highest amount of damage - if you are healing, that should NOT be your main goal. Your goal is to keep everyone alive - and it doesn't require you to have the largest healing output to keep people alive - it takes reflex, perception, and situational awareness.
All these issues boil down to the main point that I have. Raiding is different than leveling, or PvP, or even five mans. Moreso than any of these other activities, a raid needs to be the strongest it can be on a group level, not on an individual level. Balance is key. Discipline Priests may not appear by numbers to be the best option for a healing slot in a raid, but they complement the other healers very well. Disc priests help other healers by protecting a player long enough for the big heals from another healer to land. A disc priest, coupled with a pally or a druid, makes a very strong Main Tank healer team. 5 pallies as your healers will not make a strong healing team for a 25 man raid - even though they may have the highest healing output. It's about smart healing, not large healing.
This also goes for DPS. 15 Mages do not a strong raid make, even if every single one of them has amazing dps. There are a lot of spec combinations for dps players that may seem to gimp their damage output on an individual level, but because of the buffs and debuffs they provide, they make the rest of the dps in the raid stronger.
I've pointed out before the problems with theorycrafting, and this is especially the case when raiders apply it to an individual layer. One thing I will say for ElitistJerks, they always try to include various expected raid buffs when suggesting stat numbers to players - and this is how it should be for a raider. We get so caught up in the numbers, because we can actually run them, that we don't consider how crucial overall raid synergy is to the success of the entire raid. It's a team effort, and a team built of 5 strong independent and individual players does not necessarily equate to a strong 5 man group.