Comment 3 for bug 1384286

The cache (~/.cache/) directory is not equivalent to the data directory (~/.local/share/), because it's meant for volatile data which could be deleted at any time, and which can be recreated later. So, you wouldn't use it for storing user-generated content such as drawings or documents, because they could get lost.
On the other hand, I like the suggestion of sharing the cache directory as well, because that could be used for image thumbnails, which could be shared between an app and a scope.

It may also be that the application and the scope want to share some configuration values, so I'd suggest sharing the config directory as well. :-)