[jessaustin]

ISTM semantics could have been added as a supplement to PDF à la microformats for HTML, which wouldn't have hurt anything. It's easy for processors to just skip some well-defined tokens. Of course, few producers of PDFs for public consumption would have incentives to do that, so it probably never would have taken off...

[maxerickson]

Just look at how popular it is to embed/attach arbitrary files to pdf documents.

(it's well supported by Adobe tools...)