It is not necessarily true that some of the fluid has never been changed. The reason that you drain the torque converter as well is to get more of the dirty fluid out of the transmission.
The fluid goes out the torque converter, into the transmission cooler, and then back into the pan and through the main transmission parts and back into the torque converter in a continuous loop. The torque converter should hold about three quarts. The fluid goes out the torque converter and is replaced by cooler fluid at a rate of about 1 quart per 20 seconds or so while you are driving. This means that about every 1 minute you have the car running, there would be a complete change of fluid in the torque converter.
That being said, there is a reason there is a drain on the torque converter, so that you can get more of the dirty fluid out, instead of having the clean fluid diluted with some of the dirty fluid. A lot of auto makers do not have drains on the converter, Mercedes was one of the only ones to do this, and I have recently learned that there is no longer a drain on the torque converter.
I recently had the tranny fluid and filter changed on my "permanent fill" 722.6 transmission and the torque converter was drained as well.
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