That'd be one sword type from one discipline. Not true of all. Chinese and other martial arts, you hold it high for the same reasons others have mentioned. Besides, any blade to blade contact would require a high hold, unless you have an inhuman/unbreakable GI Joe Kung Fu Grip™.

 

Source: Me, been training in different styles of kung fu, tai chi, and other martial arts for 20+ years.

The problem is that the "ninjatō" as a sword is a fictional weapon. "Fictional" meaning: "there are no historical sources before the 1960's that even depict them so it's anyone's guess as to what swords ninja actually used". Most guesses are that it refers to a short sword used by ninja so how anyone would wield this is anyone's guess.

Then again "ninjatō" also literally means "ninja sword" and let's be honest here, an assassin planning to use the sword would use whatever he/she is comfortable with concealing and using.

Anyway that said pretty much every single depiction of a "ninjatō" has the grip high in a standard grip. In a reversed grip, the grip is still kept at the top of the hilt . Kenjutsu and Battoujutsu both have some very good examples of this being used in martial arts and the swords tend to be moved in arcs. The two handed reverse grip used by Ahsoka Tano in the Clone Wars cartoon also really does exist as a two handed fighting style in German sword fighting manuals. Yes every single depiction of it has the grip of the dominant hand being at the top of the hilt.

Where a Ninjato is concerned...those who insist you hold your grip choked up on the blade guard, just be really glad you would never have to have a friendly match against me. That grip of yours that you claim you have been trained to use would allow me to cause you some serious bruises with a Kendo Stick. I promise you, you were taught very wrong...obviously by people who themselves did not know what they were doing.

One inch space between the top of your hand and the blade guard. Otherwise against somebody who really knows what they are doing, that grip will cost you dearly.

Where a Ninjato is concerned...those who insist you hold your grip choked up on the blade guard, just be really glad you would never have to have a friendly match against me. That grip of yours that you claim you have been trained to use would allow me to cause you some serious bruises with a Kendo Stick. I promise you, you were taught very wrong...obviously by people who themselves did not know what they were doing.

 

One inch space between the top of your hand and the blade guard. Otherwise against somebody who really knows what they are doing, that grip will cost you dearly.

Here you go...took me some time finding this one but,

A blade made of light has no weight, so your comparison really has no standing

It does when you think about when your blade will hit something. The more obvious situation is a duel of lightsaber wielders. When the two blade will hit together, they will stop each other and each of the wielder can put pressure on the opponent blade. In this situation, if you don't size your hilt correctly like a tradittional sword, there are chances that the opponnent will make you lose grip and slash you (or at least return your own blade against you causing you severe injuries - the joys of a blade who don't need much strength to harm people)

And it true even when simply cutting things. People tend to think that when it cuts something there no resistance from the cut material. So why is it hard for Jedi and Siths to cut through a heavy steel door ?

Here you go...took me some time finding this one but,

from 30 seconds to 38 seconds in, you will see a distinct 1 inch gap between the hand and the blade guard.

Surely you realize that there are several proper ways to hold a sword. This particular niten grip is not meant for hard contact.

You can tell I'm bored today because I keep thinking about this.

The way the hilt is held is also highly dependent on the fighting style/form employed.

1. Dooku was basically a fencer, his saber hilt was curved and he rarely ever handled it 2 handed. He held the hilt along the curved portion for better accuracy/maneuverability.

2. Ahsoka Tano dual wields with a shoto style blade in the off-hand. The hilt on that saber is much shorter than the traditional hilt because it will never be held 2 handed, and she holds it further down the hilt as well.

3. Kyle Katarn uses a claymore style blade / hilt with extra length. He isnt' much for flourishes and twirls, his style is much more about blunt force trama, as odd as that is, so he holds the hilt further down as well.