Well, in FFXI, it'll be more accurate to describe the ninja as a "blink tank", rather than an evasion tank. In case you've not played the game, FFXI's ninja is capable of casting "shadows" of himself, each of it would disappear after absorbing a "hit". So, in that sense, the ninja can control when to dodge an attack. Of course, a ninja that's excellent at evading attacks would be able to keep up his shadows for a longer time, so evasion remains a very vital stat for the job/class.
The issue with an evasion and/ or blink tank in this game is that as gear gets better and better, the tank will be getting hit less and less, which would make them the far superior tank unless SE specifically designed encounters to favor one tank over the others (high accuracy fast attack speed to favor WARs and PLDs, or super hard hitting slow attacks to favor NIN). Additionally, if this were the case, parties with Ninja tanks would be far superior because the healer would be able to practically ignore their healing duties in favor of DPS'ing which would vastly decrease dungeon run times.

The other option is for them to implement an ability that spreads damage out on NIN tanks so that when they do get hit, the damage is spread over time so healers have time to react, rather than the tank going from full to dead because of an unlucky streak of no dodging.