Currently when you shutdown a host all of the sessions on that host would be dropped and the users would have to login again. For large environments you will often need to restart separate hosts due to cumulative issues, necessity to adjust sm configuration parameters or SM host maintenance.
Such single host restart of horizontal scaled environment should have minimum impact on end users. Currently unexpected drop of the operators sessions would result in data loss and negative experience with SM (there is broadcast mechanism, but it isn't always sufficient to ensure all operators on affected host will be in time to save data and logout from SM, also new sessions may be redirected to affected host after broadcast message).
I'm suggesting to consider design of new shutdown parameter like sm -shutdown -keepsessions to allow to transfer existing sessions from a host where shutdown is initiated to other available hosts.