Ya like people said, tank stance is seen as a waste because ultimately tanks are losing roughly 25 percent of their total, potential dps. Plds lose their auto attack bonus plus theres the 15 percent dmg reduction with shield oath, not to mention for pld they need gauge to use shelltron for shield swipe which build with autos in sword oath but needs to be blocked in shield oath. WAR loses fell cleave for inner beast and loses the 5 percent damage bonus in deliverance plus 20 percent dmg reduction in defiance. Drk is a bit harder to figure out due to their resource management but still they lose out on blood weapon.
SE has tried to make tank stance a bit more attractive with shield oath giving 20 gauge for a pld, Unchained which removes the damage penalty of Defiance and more self heals, and DRK getting more mp from siphon strike and bloodsplitter getting grit potencies to cancel out the 20 percent dmg penalty.
Edit: however, it ultimately depends on the party and the content. Example in current savage now, there is little to no need to use the tank stance but in ultimate bahamut, any tank tanking him is almost always in tank stance during that phase or the tanks going back into tank stance to grab aggro if there's an aggro reset.
