Reason behind using AC_PATH_XTRA?

[Jan de Groot <jan jgc homeip net>]

What's the reasoning behind using AC_PATH_XTRA in configure.ac files
used by some GNOME projects? Today I updated our at-spi2-core and
at-spi2-atk packages in Archlinux, both packages linked to libICE and
libSM when present, but both didn't need those libraries.
After some investigation, it turned out these libraries are added to the
dependency list by the AC_PATH_XTRA macro. This macro looks plain stupid
to me, but people might have a reason to use it.
In the modern world of modular X and pkg-config, is it still required to
add that macro to add random dependencies on random X libraries?