Steam itself is sort of a package manager, so that's an interesting edge case...

However, I think that there are basically three categories of application data

1) Documents -- These should never be deleted and are not invisible 2) Settings & other small data not worth deleting, probably nice to keep around in case you ever re-install. Most stuff. 3) Large semi-temporary files, like samples and other downloaded add ons that are optional parts of the application

I think OSX handles 1 & 2 well, but you're right, it needs a way to handle #3 too. However, I think that #2 is a much better default than #3.