This quote wins the day. Outside of cutting edge content, it's much more preferable to have someone play what they enjoy playing and are most familiar and effective with, even if they could potentially learn a 'better' class. You can drive shit into the ground real quick making someone play a class they hate for a 5% or less efficiency gain outside of Mythic.
If only I could be so grossly incandescent.
Tanks per say aren't rare, but thing is, most guilds want more from their tanks than from ordinary members. Tanks need to have close to 100% attendance, no lag / dc problems, speak on voice comm, often call out stuff and help raid lead, cooperate with the guild about things like positioning instead of "I do whatever I wanna".
In your random "semi-casual semi-hardcore" guild you can live with a dps who "has no mic" and "can't come every other week due to work", you just rotate dps from the bench, or a guy who pushes high dps but can't do a special task for the love of his life. With tanks, this doesn't pass.
Sadly most guilds concentrate on stuff like "best class" or "best geared" and then get tanks that show up for 3 raids then vanish and cripple a guild with constant re-recruitment of the tank until they force one of their dps to reroll. Saw this so many times it's not funny.
The most important quality of a tank is that he's reliable and responsible. Not the class he plays. Unless you're a server top guild where the first comes as granted. But if that's the case, you aren't the one asking for advice, you're the one shaping the meta and making others come to you for advice.
Can't speak to Monks, but DKs had a pretty nifty nerfbat for the middle of Wrath, which most people agree was to reduce the amount of DKs floating around (which was throwing out groups).
They started strong, were nerfed in Ulduar, and then found their legs again during TOC (ArmorPen Blood DPS).
I may pay my subscription every month, but I don't lose sight of the fact that the other 4/9/24/39 people I'm grouped with pay too.
Monks weren't actually weak at all in ToT (and I am unaware for previous but given how bad ppl were playing WW I'd chalk it up to L2P issues), they just went under the radar as paladins were immunity cheesing mechanics.
In SoO they showed how ridiculous they truly were straight up - on multiple fronts.
I think all the tanks are looking viable. Only 2 things I would consider if your all about progress.
1. Def not 2 tanks of same class.
2. What armor type based on entire raid comp. If you've got 2 rogues, 2 demon hunters, a resto druid, a WW monk MW monk, then you probably dont want to do a monk,bear, or DH tank.... Just realized how much leather tank there is wtfax.
It's not "sad", sad it would be if the outcome was decided by rolling a fotm class (or not) instead of player skill, effort put in developing the character (min-maxing), group coordination etc.
Picking a "correct class" to have an "I win button" is the definition of sad.
Wish they put the same effort in balancing other roles, it's a bit lame you have a "go to" healer (= resto druid) and a better dps class (=mage), while some are really underwhelming. Glad I play a tank I guess...