[sfopdxnonstop]

You asked: "Is this common enough a use case?"

Then you answered your own question: "Plenty of applications do just this today".

So yes, it is a common enough case that it could/should be built into the OS.

[theamk]

Well, OS could certainly offer some support, like file change detection, but the main indexing is often too application specific.

Photo albums wants to do face recognition. Music player wants BPM detection. Should those be done by OS? I do not think so.