> filesystems and printers are already easily shared (and shared through a number of different mechanisms), why would your display protocol need to handle them?
Filesystems (and printers) aren't otherwise *easily* shared from client to server, in a way such that the app running on the central server can access your client's filesystems and printers.
Of course you can turn on NFS on your client, set things up so that it's securely and encrypted, become root on the server, mount your client's file share...then undo all that again when you stop using the remote app. But that's anything but easy. MS Remote Desktop makes it *easy*. (I suspect that functionality is not actually part of the display protocol, but rather just using a tunnel through the same encrypted connection, but still it's very nice for the client to handle all of that)