From: Saašha Metsärantala
> I also wonder why not use xml:id instead of MADS's own ID.
It's not MAD's own ID, it's xs:id. xs:id is used in a MODS or MADS record when it is encapsulated within a wrapper record, for example METS, to allow the excapsulating record to link to an element within the encaplsulated record from outside of that record. It does so via xs:idref. xs:idref and xs:id work together. Using xml:id instead, this mechanism wouldn't work.