I don't think I would use that feature, because I don't personally draw a line at a certain minimum level of exposure and try to stick with it, but I would like an option for the pupil size to stay the same when adding/removing TCs with the same main lens. If I happen to touch the Av control when I have a TC on a wide-open lens and cause the camera to think I am setting the Av value, I don't want it stopping down when I remove the TC.
Well, the adding/removing TCs is what Canon says in the manual. I use it just for zooming. And if you think it's to maintain "a certain minimum level of exposure and try to stick with it" you are absolutely missing the point.
Look, here's what I use it for. I'm shooting a Heron, say. I'm close, as I usually am with these birds. Then another flies in and I need to zoom out so both birds fit within the frame. Zooming out means the aperture opens up, so on my RF 100-500mm I go from 500mm @f/7.1 to say 350mm @f/5.6 or whatever the number is at that focal length. So now I have to adjust my exposure because too much light is coming in and the exposure that I set is now off. This little setting takes care of that for me as it does it automatically. So I go from say, 500mm @f/7.1 @2000 ISO to 350mm @f/5.6 to 1600 ISO. I shoot in full manual mode and this is the only AUTO setting that I use. And I can still change all my exposure settings on the fly as always, which I do all the time and then the new settings are a new starting point.
Or picture this: I'm shooting birds in flight. I am fully zoomed in at 500mm. My exposure is perfect. Then the bird flies closer and closer and I need to start zooming out, which opens up the lens, which necessitates a change of settings so as to not overexposure my shots. See what I mean? It's just very handy to have the camera take care of it.
