Ninjas pown because they can, use awesome techniques in movies and tv shows!

They can also communicate with deaf people by signing, they rule at this.
Ninjas also have nice weapons, it has been proven that katana is way better than the swords the western countries used. Also pirates have one chance to shoot with the cannon before their dead. Its unlikely that they would hit the fast moving ninja.

Then to finish there's of course this AWESOME SONG...
[youtube]http://www.youtube.com/watch?v=-VILgSsesD0&feature=related[/youtube] (hope the youtube thing works....)