You can't lock the converter when accelerating because that's not a constant speed you'll stall the motor under the torque load, and to accelerate you need to overcome momentum. Neither drag nor rolling friction are anywhere close to high torque.
And I have no idea what the previous poster meant with a transmission cooler, I guess it's a different thing because a torque converter very much is not a transmission, if you want to compare it to anything then to a clutch. In any case I've got that explanation from an actual American actual car mechanic and random lemmings aren't going to change my mind especially while making no mechanical sense.