you dont NEED nin for a sub on rng, but its voted by many to be the best for a number of reasons.
- rng your most likely going to be puller, and rngs dont have uber defense like a war or pld, or the better defense a sam puller would have. so the use of utsumei would help save your life on a pull.
- come on your a rng, you KNOW your going to be pulling hate, if you have the gear to hit. I played 33 lvls of rng and every 1-2 hits i was able to pull hate unless a good pld tank. so if u are on with arrows and pulling hate that will help save your life again.
- equipment, archer knives I LOVE THESE THINGS they are a must have for any rng that hits lvl 28, +1 agi, +10rng acc. or +5.. i dont remember =P but they're good, wiht nin subbed you have the ability to wield 2 of these, seriously increasing your stats of acc. and agi. must have's for a rng.
so no its not the only sub you can have, war for power, thf for evas. and similar stats to nin.... nin for dual wield, utsumei, and higher agi, or even sam at higher lvl's for even more uber tp build... the sub is based on what you want and how u want to play. So again no rng does not NEED nin for a sub, but its rated the top to use given its stats and bonuses.. but again what you want to use and how to play your character is up to you.