Disc spamming one button was an ICC thing, and kind of broken. Not sure when you started playing, but disc went in to wrath as a tank healer that was more complex and versatile than a Paladin.
I can say with certain that if you had a Disc priest healing the tank you were doing it wrong. They were best as raid healers because of bubbling, and I do remember that for a very short period of time were they deemed 'kind of' good at Tank*** healing (Pre CD nerf to Penance).
that was the reason for the nerf. at the same time you were healing a tank, bubble gave you a super-hasted prayer of healing to throw out in between. the end result was the ability to solo heal 10 man naxx, and heal about half of a 25 man naxx raid including the tanks, at the same time.
but no one remembers because it was wrath naxx, it was easy regardless of who was healing.
You should read the whole post. It outlines the way Paladins have become more like Disc. Paladins are becoming more versatile, and Disc's role is becoming quite narrow.
That's like saying the class Priests aren't flexible or versatile overall. Don't think about the specs for a moment, a stand alone priest still is the most flexible healer. Regardless of a priest being in a certain spec, I think it's very hard to say "Oh well because of this spec that means that I'm not as flexible anymore!".
disc hasn't been flexible for awhile. blizz's solution to grace being OP was to make grace worthless, and turn the class into a bubble spammer from ulduar forward. that was their idea not ours. only at high levels of gear and minimal numbers of healers per raid later in ICC did getting away from bubble spam make sense, by using prayer of healing more, and even then it was questionable.
Disc is balanced around around a 24% buff that can effect only one target at a time and Aegis on PoH casts (if I crit do I get 2 Aegis'?). Its clunky and full of patchwork fixes (kind of like beacon was a patch for Paladins lack of AoE heals).
If you were able to put that buff on multiple people it would be drastically OP to say the least.
it depends. what is your definition of OP?
i see nothing wrong with making grace a self buff that boosts potential prayer/mending/absorb HPS to that of a resto druid. they clearly didn't have a problem with a 2 button class outperforming resto druids in ICC, and they clearly now don't have a problem with a 3-4 button resto druid class outperforming shaman and holy priests at 85 from looking at the beta logs.
either way the class has been screwed by grace since that pre-ulduar nerf and it looks like that won't change.
a much better design, imo, coming from their chakra idea with holy priests, would've been to make grace the disc chakra. have it pump up absorbs/mending/prayer and be maintained by prayer casts as a self buff, which is lost if you use penance/flash but penance and flash would retain their boost on targets lower than a certain percentage of health.
that way disc could remain in its current role, as a raid healer/mitigator, and would be available as a tank savior with penance/flash at a penalty. that's the current playstyle of the class, i don't see why they are so intent on changing it, except for the fact that paladins are still grossly overpowered even with the nerf to beacon, and they feel the need to try to tell people that holy paladins aren't still a reserved raid slot when the facts say otherwise.
it's a simple matter of numbers, after all..
at any given time in wrath paladins were the #1 tank class, the #1 healing class, and the #2 melee dps class, for pve purposes. not only that but all 3 of those specs were also viable in PVP.
rather than deal with the multitudes of whine should paladins and druids suddenly be anything less than the best classes in the game, it's much easier to completely change another class and use it as a (false) example of how paladins and druids shouldn't be stacked in raids.
the basic flaw in all of these design questions is that there are two classes which can effectively perform every role in the game. druids can even do it from either melee or ranged dps. that was alleviated at 60 by simply making those specs worthless for pve. their desire to make bears, prot paladins, boomkins, and ret paladins viable in pve has broken class representation. for every priest or shaman there are 10 paladins and druids. and they have no qualms about taking a risk with an overhaul of the priest class that completely turns the playstyle upside down, if it means leaving paladins and druids in the 4 button/win state that those 10 players of those classes expect.