-1 - I think Matthias covers the basic reasoning well at:

 https://lists.fedoraproject.org/pipermail/desktop/2014-October/011032.html

Beyond that: the vast majority of applications available for Fedora will
not be installed by default - it's not much of a carrot to tell an upstream
application author that they need to conform the basedir standard to be
included by default, when they *won't* be installed by default.