XML seems really only good for RSS, say you have a Book checking site, that checks newest books out.
And you have an XML file that grabs really simple syndicated feeds, that would be used to grab all the books titles, authors, etc.
Output them on the main site, this rss feed would come from a provider, which I wouldn't know of for books.
But you get my drift, though they also associate XML as a good language for Mobile Phones, and stuff like that.
Though, as XML's main purpose, the possibilities are endless, though I wonder how you would go about importing your own custom tags into HTML documents for your own use.
"The reasonable man adapts himself to the conditions that surround him... The unreasonable man adapts surrounding conditions to himself... All progress depends on the unreasonable man." - George Bernard Shaw