I do agree, that the Number of tanks would not increase.... drastically, but I am one of those who will not play a tank because its not a class I like. I dont plat healers as I do not like targetting my own people, as id rather Atone heal or Fistweave, but those are gimped and run out of mana too quickly.
Do we need other tank options? Honestly? Yes. Even though people say "all the people that want to tank are already playing a tank class", sorry but im not, and I want to tank. It gives an attractiveness to ones that didnt have it at first. Like how locks WERE the flavor of the month thanks to KJC, now we are losing the people that cant take nerfs. But Rogues, they have a very low population. But if blizz suddenly said "hey, they are now a duel weilding tank" i am sure people that were frost dk tanks from wotlk would go to that class. Its simply the attractiveness of being able to do multiple roles rather than actually doing them that makes people want to be them.