I agree that the question you say is the most discussed in the 
thread, but the question I answered here was more technical, and from 
the point of view of experimentation, and I replied to the later.

     But it makes sense also for the point of view of managing the 
configuration. I insist that several init systems could coexist on the 
same host and for the same OS, in an even more versatile way than what 
is done currently for the DE.

     For the DE, you can choose the one in function, eg xfce4, but, even 
as root and from the console, you cannot start another one for one shot. 
Only the "default" one can be run, at least using the standard startup 
command.

     For the init system, since the kernel accepts the argument 
'init=foo', you can, in principle, configure grub to offer a choice. You 
merely need to configure which one is the default and this comes 
naturally along with grub's default.

     Didier