Well, right now, there is a doctor on board the main military ship and on the main civilian ship.  Having another ship full of doctors seems overload.  Granted, it would mean improved medical facilities, but neither Caela nor RedEve will want to be there, meaning there will be a lot more doctors as NPCs.  It would help the civilian fleet more, especially in the somewhat likely event of an epidemic, but it makes two specialized PCs semi redundant, and I'm not a big fan of that.