In reply to rslifkin:
It absolutely is true. A clutch also "multiplies torque" as it slips. But "multiplying torque" through slippage is never going to be as efficient or responsive as doing so through a gear train.
I can understand there being reasons for unlocking the torque converter under certain conditions in fully automated mode, as the computer/transmission can't read my thoughts or predict my actions. But once the wheels are rolling in "manual" mode, where I am supposed to be able to take over control of the transmission, I have yet to hear a good reason ever having it unlocked...Especially with 6+ gears to choose from.