Comment # 6 on bug 929016 from Egbert Eich

The syntax of startx is: 'startx [client [clientargs]] [-- server
[serverargs]]'.
The default client is /etc/X11/xinit/xinitrc. If another one is specified on
the command line,  /etc/X11/xinit/xinitrc is not executed unless executed from
the one specified.
The WINDOWMANAGER env variable is used by /etc/X11/xinit/xinitrc to start a
windowmanager (or desktop).

Since you see this behaviour when specifying /usr/bin/startkde as client it is
more an issue of KDE. However nothing says that the client script used needs to
execute /etc/X11/xinit/xinitrc or scripts from /etc/X11/xinit/xinitrc.d as
these scripts don't supplement but replace the default client (script). Looks
like enlightenment_start doesn't do this either.

There seems to be a file '$HOME/.config/startkderc' which might do what you
want for startkde. The KDE folks may be able to help.

Assigning to kde-maintainers.