While a light agent cannot be set as the assignee of a ticket (understandable), a light agent can be selected in the To: field of a Side conversation child ticket.  However, when the ticket is created, the light agent's name is removed because again, a light agent cannot be the assignee.  One may think he out she is assigning a ticket to an "agent" when in reality, that person has a light agent role, so Zendesk is instead assigning the ticket to a group; then, this ticket has to be manually triaged to someone else.  

It seems like Zendesk should filter out light agents from the To: field.