Exactly. So while applications today each create an abstraction (e.g. SQL index) on top of the filesystem, what is really needed to provide the "iOS experience" while keeping normal file systems below, is simply an OS-level index with metadata. I can't see how it can be preferrable to have each application have an inconsistent DB of files and their metadata, versus having the OS/filesystem provide one.
As for the mp3 scenario: for a 1Gb mp3 player its certainly doable to just scan the contents for the files/metadata once in order to show track titles etc.
The problem is when the collections of files grow by several orders of magnitude and you need instant lookup of metadata (e.g. a 1Tb photo library, which isn't even especially large these days, my camera takes 25mb photos with hundreds of pieces of metadata that can't be encoded in the filename unlike an mp3 track).