This is literally how Gathers work; They have an ability which lets them see gathering nodes (Prospect for Miner, for example), and a trait which has it so that ability automatically activates. There is literally no reason to, but you can turn it off manually by using the ability again. Tank stance could work the same, have it get turned on by default and leave it up to off-tanks to remember to turn it off.
I'd say just make the effect of tank stance the default for tanks as a trait though, and change the existing stance into an off-tank stance; Rather than turning Grit on to tank, you turn Grit on to off-tank. I think they could also do some more interesting Job specific things with that as well, rather than having all 4 abilities be effectively the same thing. For example, Warriors off-stance ability could instead be a reworked version of Ninjas old Shadewalker ability. Would be nice to see some class identity return to tanks... Only reason the stance exists in the first place is to facilitate off-tanking at this point (I do miss turning it off to maximize damage and actually having to watch enmity, though), and I think there are much better ways to do that than a simple on/off switch for enmity generation...
