The main supporting counter argument is just simple "NOT MANY PEOPLE WILL USE IT"
I think that's a pretty good argument; it's likely very few would use it, if it's framed in terms of Member storage. I just don't see it being a common thing for people to want to upload files from a game environment to their Member storage.
As far as having some kind of limited per-game file storage for stuff like savefiles, that kind of thing would be used a lot--but I could see it being subject to some abuse, not to mention developers who don't take care with their savefiles will have some awfully big ones (due to saving icons, overlays, etc.). A game with many players could have thousands of savefiles, and that could become an issue over time. Nevertheless I think there's some merit to considering that. This would be very similar to client-side savefiles except only servers would have access, and only then with the correct hub password, so that could theoretically be a much more secure way to share files across servers.
I don't see why it would cause slowdown, the hardcoded limit is like 5GB and you could even further limit that internally in Export() to no larger than 50mb or something menial that whilst it will take bandwidth, it can't be used to completely break games.