I think it was in Aion where the Templar was the designated tank class, but the difference in survivability between a plate clad shield-wearing Templar and a leather clad Assassin dpser was really small. At one point groups just werent taking tanks at all and were relying on Assassins to tank, simply because they did the most damage and it let them push encounter dps much harder.
As a Templar main, my role was relegated to throwing on a two handed sword, gearing for crit, and being a DPS/offtank whilst the assassins did the maintank job. It wasnt fun.