Say the events of the previous games were being repeated or that you and the other operators were sent into a timeline that lacked the members who were involved in those games. If you could choose what operators would handle the issues offered by these games, who would you decide? Whether they're defender of attacker doesn't matter, if they absolutely need gear they wouldn't normally have for the mission (say, gear for rappelling or a snake camera for looking under doors) then it will be provided to them. Admittedly the only other Rainbow Six game I've seen in action was Vegas 2, but I could think of four operators who could do well in the place of the four you either play or control (counting for co-op). Doc, Pulse, Fuze, and Glaz.