At the end of the day, people are going to prefer having jobs in their party that will boost their own damage. You can be considerably better at samurai then ninja, thus giving your party more overall dps on samurai, but people will still prefer having you go ninja. And samurai currently is good enough in personal damage output that unless your party utilizes raid buffs effectively, similarly skilled and geared parties will get more damage out of taking a samurai; however, if your party does utilize raid buffs, ninja is totally worth taking. It just requires more optimization from the entire party.