granted, currently the "problem" is limited to functions where such a mapping is used, but i don't see what the ability to rename your arguments buys you. you'll rarely want to change them if you are not changing the interface anyways.
Backwards compatibility. Renaming a parameter is useful, for instance, when you extend an API to allow two different parameters where earlier only one was handled. For instance, a "Stdio.File file" changing into "Stdio.File|string file_or_filename" (or whichever order doesn't trig a compile error; I don't do that often enough to learn).
Spell fixes that also don't break backwards compatibility is of course nice. Granted, people have learned not to "fix" method names ("resolv" comes to mind) and could of course learn to let them remain in parameters too, but it's nice leaving the option there IMO.