The bonus is you don;t loose anything for getting crew dmg until all extra crew are dead. Technically all ships are crewed to the maximum efficiency, adding more would only means one guy is sittign on the canon because there is no more room to help, same thing as adding more guy on a job is sometimes counter productive (look agt most City workers) as only one plumber can fit in the hole, all the other are leening on their shovels pretending to watch, etc.