In FFXI, yes, good tanks mattered -- but agro management was often less about the tank and more about the DPS riding the line between "elite" and "stupid." That was the balance, and that's what made it fun! A good tank allowed more DPS, but really agro management came down to DPS being smart and not just mashing buttons.
I'm not sure FFXI is the best example to cite with regard to tanking. I don't know how it is nowadays, but for the longest time, tanking in XI was complete garbage.

The hate ceiling was ridiculously low and ridiculously easy to hit, meaning it wasn't long before hate was ping-ponging no matter what a tank did to try to bring it back under control. In addition to this, almost everything that mattered had a means of hate reset anyway.

There's a reason why many groups in FFXI eschewed tanks and simply just followed the "whoever does the most damage is the 'tank'" mindset. There's also a reason it was often joked that the best way to control your aggro as a DPS was to let yourself get killed.