The request is to store the export stems dialogue settings per song, so every song will contain different export stems dialogue settings.
Using presets per song obviously would extend the request (not anything I need, but I can imagine its usability). Using global presets for a changing dialogue like this is very challenging for the user. But if you think it works, why not add it?
That being said, down voting a request because of implementation details would result in different request for the same thing but with different execution. I rather would add my personal extension on the initial request, so people can vote on that (too).
How can we ever create balanced feature requests together if we start down voting request based on details of execution? I think when staying constructive to reach a consensus for the ultimate implementation makes much more sense and keeps similar feature request at one place. And last but not least can make a feature request better.
Of course you are free to down vote (also without reason), that's where this system is for too