Imo staying in tank stance as OT comes with 2 main disavantages:
- OT pulling hate from you could harm the party with cleave attacks (Tex)
- The obvious dps loss that mess with dps check phases.
But being fair, also has advantages:
- Tank swapping becomes easier.
- Increases survivabilty of the off tank.

Looking the picture as a whole, its not convenient, OT staying on tank stance is a mistake and must be avoided.