But you are comparing a pure holy paladin to a full ret paladin as though they are aiming for the same slot on the team.
No, a full ret is not going to completely match a holy paladin, but they come close.
A full holy paladin doesn’t come close to the damage that a ret can do.
If I’ve got someone defending a flag, I’d rather the guy that can heal or dps depending on what is approaching over a guy that gives that all up so that they can heal slightly better.
The added CC on a deep ret is more useful in a pvp setting than holy shock or divine favor as well.
The ret isn’t fighting for the dedicated healer slot on a team. They are a dps slot with significant support spells (especially for the melee) and the ability to heal well enough if the dedicated healers are busy elsewhere. Rets have a ton of flexibility that is often overlooked.
They deal respectable damage in pvp as chunks of their damage ignore armor and it all lines up in large bursts. They can cleanse spam (without interrupting their dps) to take pressure off of healers and allow them to spend GCDs healing, they can blessing of sacrifice to stay out of CCs, they can BoP healers, they have repentance, they can freedom warriors/rogues. If the healers fall, or are elsewhere, they can heal well enough too.
Granted, it takes a skilled paladin to maximize their toolkit and not get locked into the pure dps mindset, but when you find the ones that can? They absolutely earn their spot on a team.
And overall, you’d probably take one if each, so that your group could have kings + might or kings + wisdom, and the added support while leaving room for priest/druid healer slots.