I also am of the mind that it greatly depends on your group if your 2h specc will be worth it or not. If you run with pugs all the, then you confidently can say that SnB specs are 'safer' to play, but still not better. There are PLENTY of examples where the random DPSs just suck donkey balls and you have for example enpugh heals. In that situation id rather have a good 2h tank that knows how to handle his kit, than another stone that does nothing but annoy enemies.
This whole thing now can also be transferred to a premade group. If I know my healers are solid players and i know my DPSs are for example just playing with 1 inc healdebuff and not 2 and therefore a bit more damage to the whole comp would help bypass that phase when you have no healdebuff available, why shouldnt i go for it?
Another example: Lets assume your group can kill an enemy group with having 2 full deftard tanks extremely safe, without ever having problems. Those fights may take some time if the others also have lets say 2 full deftards and their guys know at least how to play. If your group now runs with a heavy dmg specced offtank that has some more tools to make enemies more succeptible to damage (higher offensive debuffs, less defensive buff potential) and you encouter the same group you just have beaten above, you can most likely still beat them, but it wont take as much time, since your whole kit is dmg heavier. I say here most likely, because your comp is not as safe as before, but can pay off a lot more. Why? Lets say you fight with them for a few minutes with your 2Deftanks, suddenly some enemies add and the whole fight tips in the enemies favor. In the other example with the offtank they might already be dead before the other enemies add , which you can also safely finish of then..
That are just some examples, but generally saying that SnB > 2h is just wrong. It ALWAYS depends on how you will play your setup and with what classes you play. There is no way to generalize such a thing. Utility has a lot of advantages, but so does DPS as well.
Edit: took me some time to write - but as Nanji said:
In every situation you need or want more dmg a 2h Tank suits your needs.