I use sshfs in order to be able to transfer files, which has the advantage that there isn’t even any need to plug it in with a USB cable. (SSH by definition works over TCP/IP and IP works over whatever is available, in my case WiFi, but ethernet or USB will also work, and do work on the Librem 5.) With this arrangement I can just use the normal Files GUI application on the desktop / laptop computer to drag and drop etc. files, or I can use the command line, or both.
A GUI alternative would be to use FileZilla on the desktop / laptop computer. Not as transparent as sshfs, and GUI only, but does the job.
The only real justification for requiring a USB cable is if your security requirements are stringent (since it requires physical proximity).
When might MTP be strictly needed? Perhaps if you absolutely insist on it working exactly the same way as on other phones. Perhaps if your security requirements preclude running an SSH server on the phone.
but to quote more fully:
MTP is not being worked on at the moment, but may be in the future
I believe that MTP already works in Linux with Linux as the host, so it is not a huge stretch to think that there is interest and knowledge to make it work with Linux as the media device.
Putting aside the uncertainty of a prediction about the future, I think you are correct on this. Few customers are interested in this and using the SIM has many deficiencies (which I have previously itemised in this forum). However the beauty of open source is that you only need to find one dev somewhere in the world who is interested in this function, and it can magically appear upstream and thereby appear on your Librem 5.