There's sense in trying to keep their diff from upstream fairly minimal, but I can see value in having some differentiation - if nothing else, the RPi foundation probably wants to ship Pi-branded images (default background, application menu icon, that kind of thing). Also, there might be issues around nonfree requirements, although I'm having a surprisingly hard time finding information about that; I know booting the Pi requires a nonfree firmware blob, but I'm not sure whether there are any ex. wifi driver issues.

That said, yes one would very much hope that everyone can work together to make the diff between Debian and its downstreams as small as possible given the constraints that each party operates under.