I agree with what you are saying. The problem is GM doesn't agree with you. That is why they make it so hard to check the fluid. Also they state in the owners manual that we don't have to check the fluid. They say we don't need to change it or the filter. Don't need to worry about it at all, just run it till the tranny pukes. This is a huge design flaw by GM.
You bring up some good points about drain and fills, the fact that if you measure what you take out it still might not be right, due to leaks and what not. But you also have to consider GM says not to worry about the fluid until there is a problem. So if there is a leak it will drain all the fluid unless you realize it and then kill the tranny. So in reality it doesn't matter if it was leaking or not, because GM tells us not to worry about it, and doesn't give us an easy way to check the level to see if it's leaking. Sometimes you won't notice a small leak untill it's to late. To the average person this sounds great, never have to change fluid, never worry about fluid level, etc. But to anyone that has any mechanical knowledge this is a joke.
You say there are to many risks to doing a drain/fill and using the fluid that comes out to measure how much to put back in. You are correct that it's not the right way to do it but it's still better than just leaving the old fluid in and never changing it. If there is a leak it doesn't matter if you change the fluid or not, either way you still are going to be short. And GM has decided not to give us an easy way to check the fluid level and they specifically state in the manual not to worry about the fluid unless there is a problem. Which a lot of the time if there is a small leak you will not notice it till it's to late.
Thanks GM, for a crappy design.
You sound like you work at a dealership or are a mechanic? If so is there a way for the average person to check the level? Mine has a dipstick, but as you stated you have to check it at a certain temp. So is there a way to tell the temp to check the fluid? I know honda says to run the vehicle for 20 minutes and then check fluid to get the right level. Is there a time to run the car to get temp to right level? Or some other way to tell the temp to check the fluid? Thanks