To understand why they are asking to be made viable tanks when compared to Warriors we have to understand the other two primary roles in the holy trinity
: damage dealers and healers.
Both DPS and healers are group based activities. You bring along a group of healers to heal the raid. And you don't just bring along a single type of healer, you bring along different classes, each with different strengths and weaknesses. DPS is the same way. Each damage-dealing class brings along a multitude of abilities outside of simply doing damage (regardless of what they might think). With both healing and DPS, all the classes work together in a group effort to get the job done.
This is a complete reversal of the tanking role.
Sure, the tank needs healers so he can keep tanking, and of course he needs DPS to actually kill the mob for him. But unlike healing and DPS, tanking is a solo-style affair. This is true even on fights where multiple mobs require tanking. And the reason is simple to see. A mob is generally attacking a single target. Hydross the Unstable is a good example. Sure, you need about 4 tanks to do the fight (assuming you do it the intended way). Two on Hydross for the two different phases (nature and frost), and two on the adds that spawn during phase swaps. But in every case, each tank is still handling his own adds, and each add is still only ever being tanked by one tank at a time.
Now that we have the scene, here is how tanking is different from DPS or healing. If a tank is dying because you don't have the healing power, you bring another healer along. The new healer works with the other healers and the additional healing stacks. If you don't have enough DPS, you can bring another DPS person in and the additional DPS stacks. It doesn't get wasted. A mob simply dies faster.
But what if your tank isn't doing his job well enough? What if he doesn't have the life, or avoidance, or resistance, or the threat generation to beat the encounter? Sure, you can bring in another tank. But in this case, unlike the other two roles, the tank coming in replaces the existing tank. With DPS, the new DPS adds to the total DPS, the new healer adds to the total healing. But a new tank doesn't add anything other than a new body that needs to be healed.
So, here you have the problem. Healing and DPS work together to provide the needed healing and DPS. Tanking is strictly a solo affair.
So consider this very real scenario. You have three tanks: a warrior, a druid, and a paladin. All are equally skilled, love tanking, and work hard to do the best they can. But no matter what the paladin does, he is going to suffer simply because of game mechanics. He will require more gear and consumables simply to remain on par with the warrior who can still use his own consumables to retake the lead. The paladin tank, regardless of what he does, will always be behind because of game mechanics.
This leads into gearing issues: who should get the new tanking gear? Warriors simply need less gear to do the job well enough, whereas paladins have to work extra hard simply to meet certain goals warriors take for granted (crushing blow avoidance, for example). But not gearing out the warrior means you are hindering the raid when the warrior needs to tank. It's a very real issue.
This is why the hybrid tanking community is so caught up in this issue. If a paladin can't tank at the base level as well as a warrior, this creates a problem for paladin tanks in general.
Most of the controversy is a result of main tanking warriors feeling they wouldn't bring enough to the table if a paladin or druid were as viable as tanks. And this is a valid concern. But what most warriors fail to realize is the sheer number of abilities they bring outside of simply tanking. Shield wall, last stand, disarm, shield bash, commanding shout, thunderclap, battle shout, sunder armor, demoralizing shout – all of these abilities are solid abilities they have access to.
Regardless, warrior tanks have proven themselves over many long years to be solid, reliable tanks, and they won't have to suddenly give up that spot. Protection paladins are struggling in the 25-man raid world, and really need a helping hand. Maybe Blizzard could come up with a way to have the tanking role work more like the DPS and healing roles.
The solution is rather simple. Paladins have been posting for some time the issues they have with tanking. These issues revolve around lack of health, lack of mitigation, and the lack of good paladin tanking gear. Giving us the health and mitigation of a warrior, along with good paladin oriented tanking gear would go a long way in solving the tanking problems. You'd still want warriors tanking. Their talents and skills bring a lot to the table. I'd still take a warrior over a paladin main tank on most of the content I face these days. The difference is I know if I needed to, I could swap the paladin in place of the warrior, and at least the basics are taken care of.
Hopefully by now you can see it's not that paladins are competing with warriors, but rather they are competing with the tanking role in general. Tanking requires a certain minimum, and that's what paladins are looking for. It just so happens that in many cases, those minimums are so easily seen in warriors, such as health, mitigation, and itemization.
It can best be summed up as such: I'd rather have a warrior tank over a paladin because of talents, skill, or gear, not because the paladin simply can't.
If you're really interested in getting more involved with the protection paladin community, and are looking for a place outside of the official forums, I'd highly recommend checking out the Maintankadin forums. It's a great place for protection paladins to come together and discuss tanking issues as well as trading information and strategies.
During the day, Jason Lotito browses the WoW forums. But by night, he takes the form of Endure, a level 70 paladin, and faces off against the toughest bosses Blizzard has to offer with his guild at his side. He's previously played a shaman to 60 and raided Horde for a while, and is currently leveling a druid just to see what all the fuss is about.