If they had it on by default you would have aggro ping pong, and you would most likely get cleaved. Imagining alliance raids with that would be absolutely horrifying.

In current content really it should just be:
MT has stance on, pulls
About 4 gcd's in OT can put on stance
Pending on the job combination OT may need to turn it off and turn it back on to avoid stealing aggro, but can also shirk as well if needed.

That's it.