While I agree that lots of people will never be convinced to play a tank role no matter what kinds of jobs get added, I think that does over-simplify the situation a bit. Lots of other people do play classes/jobs based on aesthetics--something looks cool, so they want to try it out, or they see their character (or themselves) as better aligned with one category--being a "mage type" rather than a "melee type" for example. When I first started playing FFXIV, I played any and every caster class/job, just because I happen to like magic classes. A wider variety of options will always appeal to a wider group of people, and a mage tank might convince some players to give tanking a try if they've never had an interest in melee-style tanks before. Even if it's only a few new tanks, any new tanks at this point would be a good thing.