I don't think Imu and the Gorosei care as much about the "lesser" Celestial Dragons as people would assume they do. To someone like Imu, the Celestial Dragons are just his subjects, even though they don't know it themselves. It's all for appearances so he can rule and manipulate the World at his will, from the shadows. Them being expendable was made evident by the fact that Figarland Garling tried and executed Mjosgard for helping Sai and Leo escape.