Sorry to say but we've got a lot of ignorance in this thread. I feel many of you have mistaken PvP Tanks for PvE Tanks. A PvP tank's primary objective is to provide utility for his team. Now, one can argue what is the best way to do that but.. I feel that many of you think the role of a PvP tank is to merely survive.
With that said, Immortal Tanks are absolutely viable in DPS gear. They perform perfectly fine in regs, especially when a healer is present. I would also argue that DPS geared Immortal is the "most viable" way to use Jugg tanks in Group Ranked. So don't give me this "only viable in regs" BS.
However, I should add, that DPS Geared Immortal has a substantially higher skill cap than any other Juggernaut spec/role. Since you have to perform all the same functions any other tank would need to do while also applying offensive pressure AND having less "room for error" when cycling defensive cooldowns. Only pursue if you are willing/wanting to really push your class capabilities to the limit.