Tank Stances, Traited

[frostmagemari]

Like I sai,d if ShO and Grit reduced damage by 50%, not having your stance would make you much more vulnerable

No it wouldn't. It wouldn't make you more vulnerable. You would be more protected within shield oath or grit, but you would not be more vulnerable out of it.
Since it's always been established that tanks can survive WITHOUT using the tank stances, there still wouldn't be a reason to step into tank stance except as damage prevention for tank busters or other heavy hitting moves.

You can only make a tank more vulnerable by reducing it's baseline defense so that the enemies would hit the tank harder than they do now if they were out of tank stance.