It also gives you Utsusemi, which on pulls (especially long ones) can save your life. Also good for when you pull hate and you need to tank for a moment to let the PLD get hate back.
RNG hit things like a truck. Unfortunately, we GET hit in much the same manner.
Also, NIN adds AGI and DEX to the mix, like thief, but with the Ninjutsu and dual weild, there is no comparision.Thief simply doesn't bring enough (except maybe flee) to the table when compared to NIN. The best tools a THF has, Sneak and Trick, are useless to a RNG. /NIN is by far the superior sub for RNG.
However, for Farming, and NM camping RNG/THF is great.. you can see it, flee to it, and beat it, faster than most others can. Throw in TH and steal and you can make some good cash.