I also remembered another possible use-case: kwargs in CPython. In C code, kwargs are PyDictObjects. I suppose they are usually not modified; if so, fdict could be used, since it seems to be faster at creation.
That's an interesting idea. It has always vaguely bothered me that `*args` gives a tuple while `**kwds` gives a dict. Unfortunately it's impossible to change without breaking tons of existing code, since things like `kwds.pop("something")` have become a pretty standard idiom to use it.