Again, there is nothing in this post that cannot be done by any paladin.
The correct answer is the paladin that does both. It is not a requirement to spec Ret to be able to keep your own judgment up, and any encounter where a Ret paladin can stand in and DPS safely is an encounter where any paladin can stand in and whack away at a mob to keep a judgment up safely.
The problem with this, and other people who say a paladin would be better in healing gear whacking the mob, is time and hit%. A paladin in healing gear means he does not have melee gear, including the +hit. He is going to miss a lot on 72+ trash and bosses, and he's going to lose healing time by trying to get a whack in.
He also loses time, both with the GCD and with moving, to keep that judgement up. It's going to fall off and/or someone will die, depending on what the paladin does. What is that holy paladin doing? Spot healing or healing the MT? If MT(which is the most common), he's spamming heals and can't exactly waste time with JoL or else the tank might die. If spot healing, he can judge, but then his mana pool sits there unused because JoL/VE are doing most of the work. Why not use it to do some damage?
Everyone has issues battling the GCD. It's even more stressful for a healer with cast times. Whenever I've healed in a raid, I usually don't have time to re-judge light, or someone will die. At the start of the fight, yeah. But that's exactly when you don't really need it just yet. A paladin trying to keep JoL up and play a meaningful healer means someone is going to die, and won't do as much healing as a holy paladin totally focused in using just FoL and HL.
Is a Retadin needed? No. Does he bring value equal to any other individual member? That's what we're trying to answer, and my feeling is yes. He's healing nearly the entire raid for minimal mana cost, upping raid dps, and doing his own decent dps.
The Holy/Ret paladin can't do this, because the judgement won't be maintained. In theory it seems like it'll work, but when the tank is taking 4-5k+ hits, I don't believe it's practical.