On the basis that:
XLink is a W3C Recommendation:
that cites hyperlinking as an important application,
whose linking functionalities meet or exceed those required for HTML,
whose design is fundamentally sound.
Some W3C Document Format Recs adopt or use XLink (SVG, MathML)
XHTML 2.0 removes the backward compatibility constraint.
XHTML 1.x did not use XLink - backward compatiblity with non-XML UA's.
SMIL 2.0 did on use XLink - integration with XHTML 1.1.
...the TAG concluded that:
"...XLink should be used for hypertext references in user-interface oriented applications."—TAG message
And further, that
"...that XLink should be used for hypertext references in XHTML 2.0."
Controversial with HTML-WG and others.
HTML-WG investigating HLink proposal.