If both tank know the voke timing there's no need to do extra aggro combo in Byakko ex. Even if your group fail the voke you only need to do like one aggro combo and then have OT voke shirk you during unrelenting anguish. But still it's a good fight to make DPS and healers know that aggro is a teamwork, not just MT's job. Also as long as Shirk is around aggro combo would remain pretty useless. Because it's better to tank swap than not to even isn't needed, it makes sure both tank fully utilize their CDs rather than just burning one tank's.
