How many times do we have people uninstalling apps without cleaning up the data, only to have problems later.
There are some problems with this - if I install a camera app and take a picture, then uninstall the app, if I haven't backed up the picture I automatically lose it - no choice.
And there probably should be some way of saying "give app X r [or even r-w] access to my directory, without having to make it available to all. (A bit hairy to implement, but I'm jnot the one who;d have to write it, so I don't care.)
But to limit an app to accessing only its own data? Horrors! You'd think we were talking about computers or something.