MetaData is already built-in to most modern file systems. On OSX (HFS Plus) you have the xattr command, and there are things for ext2, ext3, XFS, ReiserFS...
Imagine managing a collection of photos (or a mixed collection of multiple file formats, like scans of receipts, and pdfs with invoices) in something like Dropbox: no access to hidden low-level metadata blocks, etc., let alone easy editing. Not to speak of cross-platform support.
File names are always visible and editable, and they’re easily parsable.
I discussed such objections here: https://gist.github.com/rhythmus/11118629#problem-metadata-p...