The New Republic should get Viscounts because the Empire gets alot of larger ships
I like that the NR doesn't have some large nearly stationary ship. Their strength lies in their starfighters and smaller faster more maneuverable ships (Assault Frigates). Giving them a Viscount simply for aesthetic purposes or to compete against an Executor wouldn't improve gameplay.
As it is, facing Executors isn't fun or even really hard. If you don't want to stand off against them you can outmaneuver them with your capital ships and use starfighters to pound them.
And as previously stated by Lord Xizer, they don't fit the current eras we have except for the Pellaeon era. By then the NR has the Nebulas, Republics, and MC90s.