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.