Thanks to my installation hassle i had an a bit close look to some
installed modules. So i saw, that with the basic install + X (i did not
install any specific desktop, but only with the --no-install-recommends
option: xorg, jwm, menu, lxdm) is already pulled in the gnome-polkit.

I do not know, if a polkit is absolutely needed, but if so, wouldn't
the lxpolkit be preferable (because smaller)?