A paladin shouldn't be replacing a healer slot because paladins are tanks - they need to focus on doing tank things, not spot healing. Evading attacks, holding hate, positioning the mobs, and staying alive should be the tank's primary concerns.

While holy succor is a nice heal, it's on a 10 second cooldown. Sacred Prism>Cure is a pathetic AOE heal. Those two and divine veil can't replace a white mage. Besides that, you can already drop a white mage in many situations. I've done coincounter and mistress with 1 whm and I've heard of people doing the Cutter's Cry bosses with 1 as well. That's all with a warrior tank.

Paladin's mitigation is fine, they just need a little more damage from their combos so they don't slow boss kill speed compared to a warrior and a little bit more HP so they can make some mistakes like warriors can. Being able to parry would be nice, but only really so the Marauder subclass would be less worthless.