I'm running two specs now, a Tower of Radiance spec and an Eternal Glory spec. The difference between them is fairly minor. My "main spec" for OMFG-TANK-DAMAGE fights has Eternal Glory. I spec out of Tower of Radiance because I put Beacon of Light on the off-tank and spam Holy Light on the main tank like it's 1999.
For fights like Beth'tilac, I wanted ToR because BoL doesn't transfer between levels. I figure if I have to Bacon the tank I'm healing anyways, may as well generate some extra holy power? And I wanted to have Pursuit of Justice, because, well, I like being fast. Perhaps it's not entirely optimal, but for the fights where I'm using BoL for Holy Power, I figure I may as well not bother with Eternal Glory. In theory, I'll be using Light of Dawn a whole lot. (My glyphs reflect these choices, too, one for each spec).
I don't know what I would do, if I only had one spec to choose to be a healadin. I feel tugged in two directions, with mastery demanding one play style and haste another. Each has it's own build and strengths and weaknesses, and every encounter seems to demand something different.
We got Beth to 1% last night but ran out of time to go in for that one last attempt. I feel pretty darn useless in Phase 2, but I think that has more to do with my being a paladin than being a mastery-stacking paladin. Hit Holy Radiance on cooldown, spam the raid. Those bubbles get absorbed, at least, but without haste my direct heals are painfully slow.
I guess, even with haste, my direct heals are still painfully slow. =P
TL;DR: Paladins need a third spec so healadins can have 2 healing specs AND an offspec. Because not everyone is crazy enough to do holy/holy like me.