The idea is about how to help cleaner applications. User could remove application, but decided to preserve config/cache/etc.

Solution is about adding new field to .desktop files, like:
1. Installed - told if app is installed or not
2. Config directory - told where config are stored
3. Cache - told where cache are stored
4. Data - told where data are stored
5. Remove date - only if removed. Told, when application was removed

Of course, cleaner app could show directories from xdg_user_dirs, but users maybe couldn't know which directory belongs to which app either by name of directory. User may don't know, which app are removed and which not. These data helps user to keep profile order. Of course, the idea requires application didn't remove one's desktop file and icon, when user removes the app, but decided to not remove all the data.

Similar idea could been created for system-wide installations, because package managers could preserve config files in /etc.