I agree that adding new tanking jobs will not cause there to be more tanks in the world, for most of the reasons folks have already mentioned, and the one thing that might get more people to tank, giving it high damage-dealing capability while not sacrificing any tanking ability, would be a tremendously stupid idea, as that would then become the only DD job people would play (or, at least, the only tank or DD job that wouldn't be insta-kicked from dungeons). That said, I'm a bit depressed at the lack of diversity in tanking and healing jobs in this game; there's only two of each, compared to five DD jobs. I'd like to see more options available.
I think it's possible to add a new tank without falling into a lot of the traps folks have mentioned on this page. The key, though, is to make them very distinct from the current existing tanks. Give them specialties that shore up gaps where other tanks are weak (and give them weaknesses that other tanks shore up, so they don't completely dominate the field). For example, it might be nice to have a tank that's not just good at handling hordes of lesser mobs, but dedicated to it - like a Brawler class that has a perpetual self-buff that grants 5% damage resistance for every mob for which he's at the top of the hate list (say, to a cap of 50%), abilities for gathering and grouping hordes of mobs, and so on, but make them only above average at weathering the big hits, making PLDs and WARs preferred for standing against the big boys.
Another possibility for attracting diversity is to make the tank job interesting in some way. For example, before Geomancer came out in FFXI, I read speculation in the forums that it could be a tanking job, using a spellset dependant on the current environment to erect powerful damage mitigating buffs, like a regenerating Stoneskin if standing on rocky terrain, or ice spikes with a potent Paralyze effect when fighting on ice. No one expects a mage to tank, and the novelty of it will draw players to the job. So long as it actually WORKS as a tank, folks will stick with it.
Perhaps Beastmaster, if it ever gets added, could be a tanking job, using the pet in some way to mitigate damage, gather adds, and so on. That could be cool.