You are doing less damage than you otherwise could while keeping hate against your warrior offtank and he is harming your raids' dps output. The tradeoff to do an enmity combo on the MT is greater than the loss of your warrior doing SE/SE. Hope this helps.
For every extra enmity combo you have to do because of an offtank, you lose >80 potency. For every SE/SE combo you have to do as a WAR you lose 20 potency. It's really not hard to figure out which is a bigger raid dps loss.