Anything you can do
For all its faults, the LFD/LFR system has revolutionized how thousands, if not millions, of people play the game. As paladins, we are well-suited for taking on the challenges of group play, whether it be via tanking, healing, or damage-dealing. These different roles, however, typically require very different gear, different abilities, and different playstyles, the combination of which has served as a barrier for those rets (including myself) who dislike long DPS queues but can't be bothered to pick up a holy or protection offspec.
Retribution has a bevy of utility spells, with easy access to both threat tools and healing abilities. Many great ret paladins use these in both PvE and PvP settings to great effect already, so why can't they queue into a random heroic or LFR and use these skills to their fullest extent? If we can perform the role well without changing specs, why is there such a backlash from the community whenever people openly discuss these topics?
I can do... more or less
If you walked into this article expecting me to take a stance against the naysayers, I'm afraid I'm going to have to Mass Dispel your bubble. There are quite a few reasons why something like this doesn't work out, but I'm going to focus on the two biggest ones (I'll be focusing on ret healers here, but the same logic applies to ret tanks as well).
1. Gear distribution imbalance. A DPS queueing as heals, staying in a DPS spec and healing only using secondary mechanics will, naturally, want DPS loot. With five specs all competing for similar plate DPS gear, a ret healer could throw a wrench in the gearing plans of an up-and-coming arms warrior, fury warrior, fellow ret paladin, unholy death knight, and frost death knight -- and that's not including the tank specs that tend to avoid avoidance and take DPS gear instead.
Of course, this argument loses weight when gear becomes irrelevant. Conveniently, though, that leads me into my next point.
2. Skill/gear disparity. Let's face it, healing a random heroic as an actual healer isn't terribly difficult. You have all of the tools you need to succeed already, with the only roadblock being actually using those tools. As a ret healer, however, you are intentionally sacrificing DPS to do a job you have very few tools for. If healing a random heroic as a holy paladin is like building a birdhouse with a hammer and nails, a jigsaw, and Bob Villa looking over your shoulder, healing one as a ret hybrid would be like trying to build the same birdhouse with three bent screws and a shoe.
In order to both maintain "acceptable" damage levels and keep the group alive, you either need to be quite skilled at balancing the two, or overgeared to the point of trivialization. At this point in Mists it's a simple matter to collect enough gear to choke a sparkly horse, and I have seen a few players in my time that astound me with their skills when it comes to juggling responsibilities.
Unfortunately though, the majority of the playerbase isn't quite as lucky to be so gifted. You can bet gold that players without these essentials will attempt hybrid play, and many will fail miserably at it. Failing as a healer is one thing -- many groups will give a troubled healer time to prove themselves capable of doing their assigned job. The same can seldom be said for hybrids.
I'm all for experimentation and unorthodox playstyles, but the simple truth is that current systems don't support such play in a random setting. It is unfair to subject four total strangers to the possibility of wiping and wasting everyone's time because one person wants to do something that the rest of the group never agreed to in the first place.
Attack the cause, not the symptom
While I don't think it's prudent or considerate for a hybrid to sign up as a role they're not suited for, I do admire the ingenuity and unconventional wisdom that hybrid play presents. I would very much like to see this promoted in the future, but for that to happen in areas like random heroics and LFR, the theaters arguably most suited for it, the systems governing these areas would have to be altered in a not-so-insignificant way such that players can come to expect a hybrid to do the job of a more traditional role.
One could argue that just because a player designates themselves a healer and specs into a healing tree, that doesn't guarantee that they know how to heal. Conceptually, a hybrid would be no different -- the trouble lies in making it socially acceptable to put a hybrid in non-DPS roles. Essentially, in order for this idea to work in the closed system that is queue-able PvE content you would need to put it back in the box it strove to be out of, a move that would most likely cause it to lose its uniqueness and flair, and we'd be back to the confined trichotomy (tetrachotomy?) we tried to shake up before.
The holy MMO trinity of tank, healer, and damage-dealer has worked quite well for the genre for years, but with a game as old as WoW you can't rule anything out. Then again, would Blizzard want to jeopardize the future of the franchise by tampering with the formula?
The Light and How to Swing It teaches you the ins and outs of retribution paladins, from Ret 101 and how to gem, enchant and reforge your retadin, to essential ret pally addons.