Okay here's a few reasons why Ninjask won't work
1) Phazers. They're everywhere. Giratina-O, Giratina, Groudon, Dialga, Lugia, Skarmory, and even some Arcei. Ninjask isn't doing anything to them.
2) Priority. Ninjask's pathetic defense means that it will have a rough time passing stuff when it could potentially be killed by priority before it passes. Arceus' ExtremeSpeed is the largest threat, followed by Rayquaza, Giratina-O (who can also Dragon Tail), Deoxys-A, and Scizor.
3) Has a tough time getting a Swords Dance boost with its terrible defense. Wobbuffet fixes this somewhat.
4) Stealth Rock. This just means that Ninjask doesn't get much chance to repeat its trick until it gets it right. Also puts it into priority kill range.
5) Gorebyss and Smeargle exist. Gorebyss can take down or severely dent the phazers (except Dialga), has a fantastic base Defense to absorb priority hits, isn't torn apart by Stealth Rock, and has the potential to sweep on its own. Smeargle can Spore potential phazers and priority users, and pass somewhat safely. Both offer arguably deadlier boosts, powering up both Special Attack and Attack for monsters such as Mixed Dialga and Giratina-O to abuse. It also gives the Baton Passer flexibility in choosing the appropriate recipient.
tl;dr
QC Rejected 2/3