It would be interesting to use layers in a file system. This could help for searching, archiving and other more complex tasks such as installing and uninstalling a software package.
For example, actually, a package installs files in /usr/bin, in /usr/lib, /usr/share and so on. There would be a folder called ‘my_package’ that would mount its own folders bin, lib, etc. into the main filesystem, along with /usr/bin, /usr/lib, etc.
Adding and removing a package would be much simpler: just mount or unmount the package.
Another use is for archiving. I would like to archive everything written in 2011,2012,2013 in several folders. In my folders, I have folders for 2011,2012, etc. Then, the year would become a “layer”. We simply could select everything in the layers of interest and operate on the selection. Example: I could save everything for 2014 into a DVD-ROM; it would select all the directories with a folder 2014 in it.