To be fair, evasion tanks are easily countered as they can't evade a meteor dropping on their head. Evasion (and Parry/Magic Defense Tanks like say Rune Fencer in XI) works fine in certain regards but the issue comes down the enemy design.

In XIV, enemies are far too scripted and have far too many 'AoE' attacks that an evasion tank would get hit by regardless, so it would actually be absolutely worthless in a game with no battle system depth. In FFXI people say Utsusemi broke things but in all reality, for as much time as you cast that as a sub you hinder your own output. SE's counter was pretty much to make bosses and the like ignore the ability so you can't try to hide behind your evasion/shadows.

It would stress the playerbase if they designed enemies that actually tried to kill you.