Ninjask is decent, it's simply that people don't use him properly. Instead of just passing one speed boost, when faced with a difficult situation, they opt to try to obtain multiple ones, thus being phazed out, or they die/waste all their health. With proper support, passing single speed boost around (and Swords Dance when you can) can be a blessing to many sweepers in OU.
I rarely see Ninjask paired with Magnezone. Just look at the usage statistics for him, Magnezone, someone who could trap and skill Skarmory, Scizor (not to mention it would like a speed boost), isn't even in the top 20 for teammates. It's clear people view him as only good on a Baton Pass Chain, which is nearly impossible to pull off in the current OU.
The fact that Starmie (the only Rapid Spinner on the list) is a number 20, and absence of Forrty is equally depressing, because they're not supporting Ninjask at all. :/
+------------+---------------
| Pokemon | By Usage |
+------------+---------------
| Ninjask | 1. Scizor
| Ninjask | 2. Metagross
| Ninjask | 3. Smeargle
| Ninjask | 4. Vaporeon
| Ninjask | 5. Salamence
| Ninjask | 6. Lucario
| Ninjask | 7. Breloom
| Ninjask | 8. Marowak
| Ninjask | 9. Infernape
| Ninjask | 10. Blissey
| Ninjask | 11. Gliscor|
| Ninjask | 12. Umbreon
| Ninjask | 13. Heatran
| Ninjask | 14. Gyarados
| Ninjask | 15. Gengar
| Ninjask | 16. Latias
| Ninjask | 17. Tyranitar
| Ninjask | 18. Electivire
| Ninjask | 19. Machamp
| Ninjask | 20. Starmie