Toolbar: site switcher: my "steven" user can access to two virtual sites but can not switch trhough the toolbar site switcher, why?

I have a "Steven" user who can acces to two site (with some RW permission on two sections of thoses two sites... the rest is visible by guest so indirectly by steven too).

Steven has no access to the site switcher in the toolbar, why? How does the check is done? Problem with the guest user which indirectly grant permission to all other users? Do we want to offer such a site switcher only for "editors" (users which can at least edit a content object on a given virtual site)? but then in such a case how do we determine if a user is an "editor" for a given site?

Please come to me to discuss this point if required in order to take some functional decision