Failed XML standards

As we all know on this list, the XML article in Wikipedia contains the
following:

"Some other specifications conceived as part of the "XML Core" have
failed to find wide adoption, including XInclude, XLink, and
XPointer."

But what does it mean?

Probably all text books about XML have a chapter dedicated to each of
those standards.

Would you say that a new basic text book about XML should hardly
mention XInclude, XLink, and XPointer at all, at least not in whole
chapters, but maybe in some paragraph in a chapter about failed
standards?

Cheers,
Jesper Tverskov

http://www.xmlkurser.dk
http://www.xmlplease.com