I've decided ninjas are cool for the same reasons that pirates are cool.
As historical figures neither ninjas nor pirates are at all admirable. But people who joke about ninjas or pirates, or who dress like them for a costume party, or kids who pretend to be ninjas or pirates -- none of them are paying any attention at all to the historical realities of assassination, government dominance, pillage, or plunder.
And that's the first point. Ninjas and pirates are cool because people are free to re-define them. Some connotations remain of danger, bravery, and skill. Everything bad about the characters can be ignored or even reversed.
The second point is that both ninjas and pirates have many elements of "mix and match adventure cool" readily available.
What is mix and match adventure cool? I just invented the phrase while thinking about the first Star Wars film, the science fiction book Snow Crash, and the anime (cartoon) series The Last Exile. All three are delightful and ground-breaking within their genre. And each makes use of nearly all of the following:
- sword fighting (light sabers count)
- a handsome scoundrel who runs his own ship, and turns out to be a softie and sweetheart
- a princess out adventuring instead of staying at home
- a young male protagonist who find out he is not the most dangerous or capable fellow out there but can still make a difference
- fighting done with small and agile vehicles (futuristic flying cars, including x-wings, count)
- a rare ability to get somewhere is vital and heroic
- honorable people suffering a setback due to betrayal, but eventually honor triumphs over corruption
- an impersonal and heartless government that is set back or overturned
There are other examples of blending elements of genres to make something cool. The first Harry Potter book used a traditional British plot of an oprhan at a boarding school and combined it with the outsides of cute Dickensonian villages and the insides of fairy-tale castles.
Still, I wonder what else will appear using the winning combination of mix and match adventure cool? Maybe the next computer game to be as pervasive as Myst once was will involve a ninja or pirate protagonist and use all the bulleted items?