Doesn’t the secure option require ports only a root user can bind too? And you can always create secure tunnels if the physical network is insecure.
Now, how about this common scenario: I want to run a file-sharing server on my network. I want a random "friend" to come over and grab a copy of a file, but I don't want them to see any other files on the NFS server.
So, the "friend" has root access on their device. They can just log in and lie to the NFS server, claim they're my UID, and see all my files that I didn't want them to access. Configuring KRB in that scenario is totally impractical.
How is that a common scenario? Why not give them your drive and the encryption key while you’re at it? It would be way faster.
The correct scenario would be to just copy the file and serve it with ftp or http on another interface.