Well, I'm not exactly sure about what level of content you are speaking.
- If you just created your DK and did the first 5mans:
Those are floated by bad DK tanks. They just left their starting zone. Most of them are horrible. Other tanks did probably tank through most of vanilla dungeons. (Well ok, paladins and warriors, it's horrible to tank those as a bear because you are missing all or nearly all moves to hit more than one opponent, so druids probably go as cat.)
- If you just reached 85:
Fresh DKs are quite hard to heal. Especially if they go for stamina and maybe even play bad. They take no hits when a CD is up or they are lucky followed with some big hits the healer has to heal through. I've seen many fresh DKs who had to be healed with (stopcasting) FH constantly, maybe with a GH thrown in, while other fresh tanks can be healed with Heal spam and some GH thrown in. So it's at least one category of heals more mana-expensive to heal a fresh (and maybe bad) lvl 85 DK. Sure DKs have selfheal, and not-as-bad DKs use it. But since the healer does not know if the DK has one ready or not, the healer has to start the expensive heal, anyway. (And maybe finish it, because they maybe have not enough time to use a cheaper one when they see the selfheal happening.)
That changer with better gear and more experience of what to do where, eventually. I think mastery helps a lot.
- If you are speaking about raid entry level
DKs are fine here. Well, not really: medicore DKs are fine here. Good DKs are very usefull. Bad DKs are nothing you want to have here. I've seen all of them at that level. The variance in DK skills and DK gearing is probably greater than with shield-tanks. I've seen only one bear so I don't want to generalize about them. The mastery of other tanks works passive. They cannot decide when it should happen and they don't have to. Other tanks have many options to screw up, too. Wrong gear, wrong "rotation", wrong or no CD use, wrong talent selection, I've even seen warriors using sunder armor instead of devastate, so include wrong "spell selection" to the mix. But at least their mastery stuff is done automatically and since they don't have as much CDs (and maybe selfhealing) than the DK, they depend less on them, so it may be less important if they do it wrong (or neglect it).
So bad DKs tanks can make a "more bad" impression than other bad tanks. And if a raid did make such an experience they maybe don't want to try out another one.
Additionally: Most guilds know that warriors can be excellent tanks. Even while they were the worst tanks on paper most of WotLK. But before Wrath, nearly all MTs were warriors. So everyone who did raid back then, probably has seen a good warrior tank. They may have seen bad ones in 5mans, but without LFD-tool they probably mostly went with those they already knew.
Most guilds have seen other good tanks in Wrath. Yes there were times where DKs were OP. But it takes time until new tanks are used in raids. And chances are high that not everyone has seen all classes "seriously" tank back then. But more or less everybody has seen a lot of DK tanks while leveling up and maybe early 85. Those probably did left an impression about DKs, too.
Back to the title-question: DK tanks are not needed. As non other tank class is needed. That's because all 4 are viable and player skill actually is more important than tank class. Good tanks are needed.
All the tanking classes have advantages and disadvantages, but as for your question; yes DK's are needed as are Pallys, Warriors and Bears. But as said above, a bad DK tank can be really, really bad. Based on my experience, Pallys are the easy mode tank in that they are very forgiving, Warriors have lots of toys to play with and are fun but I've been running across some threat issues with them of late and DK's take a lot of practice to get good at - healing, cooldown, usage, Death Strike priority, etc.
I personnally feel DK's excell best with high magic dmg fights..I myself have a dk,warr, and pally tank, along withpally, shaman, druid, and am lvling a priest healers, so ive seen from both pov's...
I think the "dks are worse tanks" come from the amounts of dmg they were taking while still gearing up..They did have low avoidances, and many ppl werent properly using cd's and blood shield..I have to agree with Prexie that , in the correct hands, they are extremely viable tanks...As they gear up however, and learn the new way to play in Cata, they can take lots of dmg, and it seems to be steady, with large amounts of it.....Using your cooldowns, and bloodshield properly can make or break the dk tank...
Speaking as an experienced Warrior tank with a (newly 85) DK tank:
The DK works well - excellent threat and a good toolset. But at her gear level - (epic belt I 'smith'd for her, plus mixed rep/dungeon drops) - she definitely feels a bit more squishy than my Warrior did at the same point. I don't mind using cooldowns, nsince that's what they're there for. So it's not "bad", just that I find myself playing at a more "measured" pace: Runes and ability CDs dictate a more thoughtful approach, whereas warriors begrudge any slowdown in pace due to rage mechanics.
I have a DK tank that is 6/12 in current raid progression and am told by my healers that I'm easier to heal than the protpally I tank with. As someone above said, playing a DK tank is more dependent on player skill than the other classes. In the hands of someone who doesn't know what they're doing...not good. But in the hands of someone who knows the class well (prioritizes Death Strike, uses cooldowns liberally), they are extremely strong.
DKs are extremely strong against single-target boss encounters because of their self-healing and loads of cooldowns. They're weak against multiple hard-hitting add encounters where their lack of a shield and block mechanic makes Pallys & Warriors far superior.
I find that threat is laughably easy on my DK tank compared to my warrior tank.
We all have our different strengths. Two examples that illustrate this are Argaloth in BH for which a DK is excelent since we can solo tank him (letting us have one more dps in the group) and Omnitron Defense System where we are quite weak since we cant continue to self heal without fear of breaking the shields.
There are many more examples between those two of course.
Think the biggest difference is that for warriors/pallies block is passive and always a set procentage of the damage 30/60% and 40%. While dks 'block' is completely in the hand of the player with deathstrike usage and bloodshield will almost always have the set amount of absorption.
This makes it more important to play a dk properly while a warrior/pally can just afk after they have threat. And i feel dks are really strong on low tankdamage fights and slightly weaker on high tankdamage fights.
Also the large amount of dk cds makes it important to play well.
I have a dk than and a pally tank. My dk is t11 4peice (normal) geared and I used to raid with him all the time. However, I recently got my paladin raid ready but have found that my guild only want my paladin even though he is far behind in gear. The reason I was given was that pales are easier to heal as they take less damage overall but doesn't the healing from death strike make up for that. I don't know if this is true or not.
But that is most people on my servers logic.
Zee786: Read our answers from above. Yes, all tanks are viable and quite balanced if played right. But the DK has more options to do it wrong than especially a paladin has. So it's easier to be easy to heal for a paladin than for a DK.