I'd also argue that if you need to turn a cache snoop into a network round trip (or several?), your abstraction is just as broken as if it returned the wrong value; it's unusably slow :)
Yes, that's why I'm still having trouble to understand the fuss about VFS abstraction as used in 9p etc ;-). I've always been glad to know when I was not on an NFS or sshfs mount (mostly for reasons that you can't design away, i.e. network reliability issues). So why bother abstracting out that knowledge even more?
When you come at it from the perspective that remote resources are the norm, and you assemble a computer from resources scattered across the network, local access becomes weird special case. Generally, you're running a file server and terminal as separate nodes, with an auth server somewhere else.
And if you're actually using the knowledge that some files are local, you get bugs and assumptions creep in, and now your software stops working in the case where you're running on someone else's network.
It's about transparently providing the same interface to everyone, and making that interface simple enough that implementing it is easy enough that it's actually done, and the interface actually gets used.
Then, if you want to interpose, remap, analyze, manipulate, redirect, or sandbox it, you can do that without much trouble. The special cases are rare and can be reasoned about.
Reasoning about your system in full frees you to do a huge amount.