I am beginning to think this would be a good addition, but for reasons not mentioned on this thread.
It would allow us to prevent 2 surprising results when managing files in Krusader, Midnight Commander, command line, etc.
If you mix user shares and disks when moving/copying files, you can actually lose data, because Linux doesn't realize that the source path and the destination path might actually be the same file, and so tries to overwrite what it is trying to read. This is often referred to as the User Share Copy Bug.
The Linux command for Move and Rename are the same, mv. So when you try to move files from one user share to another, Linux will often simply rename the files so they have a different path on the same disk, in violation of any user share settings such as included disks. The workaround for this is to copy from source to destination, so that new files get created following the user share settings, then deleting from the source.
If we had our own file manager, number 1 could be rejected, and number 2 could implement the mentioned workaround.
Of course, this would be a very large undertaking. Maybe some existing source code could be customized.