People forget that Warriors can sorta negate the damage penalty with maim (+20%) and storms eye (-10% slashing), a good warrior can pretty much keep it up and hold hate at the same time. But i think it's probably more efficient to have warrior as an offtank to do slashing debuff + attack debuff and DPS and leave the main tanking to PLD.
off topic: that said, I play both classes and it's no contest when it comes to damage, a warrior can out damage PLD easily while tanking/off tanking.