IMO it is  appropriate that:

* Slim must check whether or/not there is a monitor or multiple monitors to put something.
   Not blindly do something itself.

* if there is NO monitor at all then it simply go to idle and wait for one to plugged.
   listen for plug or check periodically.But that up to user to decide; So it exposed config option to user to what to do when NO monitor, just quit or go to idle and wait?

if slim wants to support headless and/or network login and/or forward the login window on network then it must provides related config options to its users to decide. it is up to user to decide not slim.

if slim wants to work with multiple monitor case then it is exposed related config options to users to decide. then user decides eg:

    *present yourself on both monitor simultaneously to interact/login.
    *use ID=xyfoobar  monitor
    *use internal monitor ignore others
    * ... likewise

So feel free to file a bug.