If it takes off the internet would be easier to get information from.
building catalogues from lotsa stores would be piss easy. just write a very simple query like
Get <description>PSP</description> <price>?</price> and the query would search the net for those containers and bring back an answer from all available places.
There are bunches of schemas to define data for a whole bunch of things. XBRL, MathML, etc
XML defines data, not display. so it's not like HTML which defines how things are displayed although XHTML is an XML display schema.