A common question that also follows is "Why haven't you leveled Ninja on FFXI yet?"
Answer:
1.) Ninjas are amazingly awesome.
2.) Thief is more ninja-like than ninjas.
Explanation:
1.) Ninjas are amazingly awesome.
Here's a short tale about real ninja involvement:
Tokugawa Ieyasu (http://en.wikipedia.org/wiki/Tokugawa_Ieyasu)used ninjas to give him the ability to become a Shogun of Japan. There are historical documents proving that this actually occured. At one point in Ieyasu's life, there was a certain figure who needed to die. He could have easily charged into the man's home and slaughtered him. However, if there was any evidence at all that he was forcefully killed, war would more than likely break out, and Ieyasu's dreams of power would be shattered.
So he hired a ninja from the Koga clan to "Help Him out" (Koga was a style of Ninjitsu) (the word "Ninjutsu" does not exist, SE made it up). How did the Koga ninja accomplish this task?
First of all, he had to have a weapon to kill the man with. Since no man "naturally" dies from a wound, any sort of metal object was out of the question. Thus, poisons were the only method. But how to get this man to consume a poison? "Food Poisoning" would prove difficult, as the man had another guy who would eat/drink his food before he himself ate it, to check for such a thing. Also, the traditional "Death from Above" trick (poison droplets on the string which were lowered into the victim's mouth while the slept) was difficult as well, as the man's room was tightly guarded.
What tactic did the ninja use then? Simple: Poisoned rice.
Rice is a very unique food in that you could poison a single grain of it, which would make the rest of the meal seem safe. The ninja snuck into the kitchen (which is easy enough for ninjas. Ninja Myths frequently state that Ninjas had the ability to go completely invisible) and deposited one single poison-dipped (it is unknown exactly which poison the ninja used, but it had to be a fairly strong one) grain of rice into the bottom of the bowl, so as the man's food tester wouldn't consume it.
The man died in his sleep that night, and no one suspected any form of foul play.
2.) Thief is more ninja like than Ninjas.
Although in myths ninjas are said to use a variety of magic, from turning themselves completely invisible (Tonko) to turning into animals, they needless to say did not really perform these types of duties. Ninjas were VERY stealthy, and very sneaky. The best example of Ninja stealth is shown by the Sanada clan, when their castle was attacked by an enemy force 20x greater than their own. They were outnumbered in man power and seige weaponry, and most other castles would have fallen.
Not the Ninjas.
The Ninjas used a very risky, but tricky, strategy: They opened their castle gates wide open and let the opposing army in. In the meanwhile, all of the ninjas gathered at some point in the center of the castle, extinguishing all the lights and covering all the crevices in the walls to bar light from coming in. This made the inner castle extremely dark. The ninjas then hid all throughout the castle, bows and wakizashis in hand.
Now, if you were an assaulting japanese army and you saw not only the ninja's castle gates standing wide open, but a dark luminous complex with all entrances sealed but one, what would you do? That's right, you'd be scared *********
And that's exactly what happened with the opposing army. The ninjas played it very smart in that they did not shoot the men right as they entered their dark castle of death: they let them walk around for awhile. One by one, the Ninjas picked off the enemy soldiers, going for the ones carrying torches first. Since the ninjas were in full black in a pitch black castle, they were basically invisible. The vibrant colors on their enemy's armor were like bullseyes to the ninjas.
Needless to say, an army with a 20:1 advantage quickly became a 1:3 disadvantage. The siege equipment the enemy had brought was completely useless, as the castle gates were not wide enough to fit any of it in. Also, the enemy's brain dead commander kept sending batallion after batallion into the castle of death, with the sounds of blood splattering seconds later. When night time arrived, the commander could see his attack was futile and ordered the retreat of his army.
Then the ninjas shut the gates.
What few soldiers who were outside of the castle were shot by arrows from the ramparts of the castle.
The remainder of the enemy force was now outnumbered 3 to 1 in a very cramp environment, at night versus a ninja army.
The slaughter began.
The ninjas had every advantage possible: It was night time in a very confined space. The commander had sent in the brunt of his army into the castle of death, so the remaining one thousand soldiers were all of simple nature, like the men who operated the siege equipment. Needless to say, they had little to no actual combat experience, and it was basically like putting 1000 sheep versus 3000 wolves.
The Battle of Sanada castle is a true attribute to Ninja Awesomeness. It is quite possibly one of the most remarkable war-strategies ever employed, and it was done by none other than the ninjas.