Holy isn't balanced.
Holy is balanced by its tree of talents and its gear, in the same way prot and ret are. There is nothing realistically in the base paladin class that makes us great healers (oom, no regen etc) it is all typically provided in the talent trees.
Spiritual attunement isn't a kludgy mechanic, if anything its a perfect synergy talent:
When in combat, taking damage, Paladins have effectively unlimited mana.
This means that as MT, during splash damage etc we it is a great mechanic for raiding, however for soloing and non-damage situations each class needs its own booster. Blizz said that the effect on Holy of massive mana regen from splash damage was bad, in truth this reflects badly on the raid design of "hit everyone or its not hard" rather than the mechanic itself, if there were fights more like patchwerk or targetted damage fights then SA would be a lot less effective and fit more with what its meant to be, a tool to make us more effective in combat (look at WC3, a Paladin was virtually unkillable if supported correctly, it is a good mechanic for that).
In terms of divine plea, it is broken because it tries to fix the flaws of 3 different classes within 1 base class. What should be is 3 separate mana restoration effects, 1 in each tree balanced to that tree. The kludge was including something they didn't want a spec using in the base class without adding a significant toggle to it to make it optional (for example, shadow form toggle is optional, shadow priests don't need to cast holy spells to do damage or regen mana, ret prot and holy need to regen mana... the kludge is to try and fit it to all 3).
And we do heal while tanking, Naxx 10 / 25 I heal while tanking the Horsemen for example because I don't get a healer. What stops us doing so is that normally the push back / heal is irrelevant compared to the loss of threat and additional damage taken from melee bosses (spell bosses we can heal with few concerns normally, I healed myself through Leo demon phase as well and Illidan Demon Phase when I decided to tank that too
).
Divine plea is the kludge as it doesn't fit:
SA: Infinite mana while MT, extra mana if subjected to raid damage
SoW: extra mana to help
JoW: raid mana
xxx: Holy needs something akin to divine plea, given the inability to realistically stop healing (and with the nerfs to oo5sr this view might change) means they need an active regen, divine plea similar @ 15-20% and -20% healing would likely work.
xxx: Ret should have its GCDs filled to make DPS / Healing a trade off and minimise burst. Another ability or expand JotW.
xxx: Prot needs either active or passive regen to enable tanking while in OT role (no damage), and to enable sustained functionality, best bet is likely an ability like JotW but as a strike on a 9-15s cooldown that must be weaved into the 969, in general prot could do with a few more strikes to make it more interesting.
Stances fix nothing if the base class is broken, and thats the problem, ShoR and Divine plea are realistically tree specifics (Prot and Holy most likely) rather than full class solutions, thats the problem, we got stupid abilities that don't fit the class as a whole.
Better choices might have been:
11 point prot: ShoR (yes accessible strike at low level zomg)
level 75: Active usable ability: new strike based on SP or AP, ShoR is too specific to prot to really be usable by ret or to a lesser extent holy.
level 80: Active heal / shield (sacred shield type or a proper bubble) -- this should be a raid type skill suitable for all classes, so a large xor scaling on SP or AP as a personal or targetted shield (likely personal) that has an effect if it wears off (regens % shield x 100 mana or similar).
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