Actually, Holy is less versatile than a properly specialized and decently geared Ret Paladin. For example, if you go deep in Holy you end up with Holy Shock, which is okay, but not great. Even with + healing gear you will likely only get 1 critical heal with Holy Shock every 4 uses. It simply doesn't heal enough to make any difference. I mean, even if you get a normal heal for 750 (despite the fact the tooltip says less than 300), it does nothing considering the amount of damage an FC receives from numerous sources. When you factor in the time for Holy Shock to land and any global cooldowns, a couple seconds has actually passed. You would be better off trying to land the 2.5 heal for 2,500.
On the other hand, Repentance provides an extra CC, and any experienced WSG PvPer knows that the game is won on CC, never damage or healing. Most CC wins, hands down, every game.
I do acknowledge the importance of healing in PvP, and for the Retribution Paladin to continue the roll of support. He should spec about 20 talents in Holy, and 31 in Retribution to be an excellent support class overall. Gear up with plenty of intel to increase the chances of the holy spells landing as criticals, as well as having plenty for healing. A Ret pally should aim for about 5k mana unbuffed.
A Ret Pally should never consider himself to be "a warrior who can heal." That is not the definition of a Retribution Paladin. The very word "Retribution" means "punishment inflicted on someone as vengeance for a wrong or criminal act." This means that if a Horde attacks the Paladin or a teammate, the Paladin can strike back using any of his abilities, and the method he employs in striking back could also be how he buffs up and/or heals a warrior to terminate the enemy.
Also, playing a Retribution Paladin provides the paladin the best means of self defense, as opposed to Holy. Holy is useless when spell-locked, and cannot heal himself out of trouble, nor can he melee decently to defend himself. But a Ret Pally who had SoC active can actually fight back.
I noticed how people seem to think that Holy is some kind of pvp god, but the reality is that Holy only sounds like a good idea because of how much people want heals. However, Holy cannot heal any better than any other Paladin spec when he is being constantly CCed and spell locked. Full Holy has no more defenses against CC than a Ret Pally who has 20 talents in Holy, and Full Holy is actually more defenseless.