There's no concept of "external". What would it be, "USB" or anything mounted under /mnt or /media? What if it's the root OS drive of another computer you're trying to fix connected through a USB-SATA adapter? Should any program running with minimized privileges get to overwrite even root files in that OS drive?

I think that it's a pretty good heuristic that if permissions exist in the filesystem, they matter and shouldn't be ignored.

They shouldn't be ignored. but they can be ignored, is the problem. File permissions are not encryption or security: If I can't read a file on this machine, because I'm not root, I'll just move the drive to a different machine where I am root.

But I agree with you, they do have a use and to some use cases matter, and we shouldn't arbitrarily decide to ignore them.