Even in battle spot you dont have the luxury of taking five turns to bulk up. Considering even in doubles or even singles fire or rock could Ohko or 2hko. Even if you go protect, (speed boost) Swords Dance, if you dont get burned off by a decent flamethrower you should use baton pass to have +2 Spd and atk upon another poke. Best case scenario you will have 1Hp with sash or up to 20% of its health left if will be almost as if it were dead because you would be unable to do the same set again. Even if you place a sitrus berry you are maxing out at most 50% at best, which isnt much with a frail body. Frankly there is no good reason to use ninjask unless you like bug pokes and you dont mind losing. Dont believe me? Ask Griffinbane or PlatinumDude.
I maybe more of a newb to the competitive thing but I do know enough not to use ninjask
Ninjask is too fast for any rock or fire type to get a hit in on it even without giving it any speed EV's. Ninjask is actually one of my least favorite pokemon because I despise how weak bugs are, that's why in my team I have pretty much all bulky slow pokes. But it's undeniable that it has its uses. When I do online battles Ninjask works perfectly fine with leftovers. I use Substitute right out of the gate and they usually either switch or use spore or some other such thing which gives me extra turn to use. Then I use protect to heal a bit, then the next turn Swords dance. Which by this time Ninjask has +2 attack and +2.5 speed which makes it perfectly fine to switch out with something else. The only
real threats to ninjask by this time is maaaaybe taunt or a pokemon with the infiltrator ability. But ninjask is too fast to be caught by taunt so the only real threat is infiltrator. Which is why you plan out when to use your pokemon so you can take out the threats first.