Each Jedi uses a lightsaber that works for their own preferences.
For instance, Asoka Tano (in The Clone Wars ) handled a lightsaber differently than most Jedi .
She originally used "a reverse Shien style grip."
And later "she crafted a shoto" to use with it.
(The "Shien" grip is holding the lightsaber so the blade is on the side of the hand with the little finger, instead of with the thumb, as is usually seen.)
The Jedi recognize a number of types of lightsaber combat and each Jedi will work with a style and grip and lightsaber that works best for them.
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Every difference in the configuration—even the length of the saber—produces a different set of advantages and disadvantages.
A double light saber has the distinct tradeoff that pointing one end toward the opponent risks the other end hitting yourself.
Likewise, a single saber of say 20 feet length instead of the usual five or so would be a disadvantage in close quarters: striking the ceiling, walls, etc., causing structure side effects as well as draining energy from the weapon as well as the user.
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As an Epee fencer in the real world, I favoured a weapon with the traditional French style grip (long handle, heavy pomel).
Many people, including my old coach, consider the more modern "pistol" grip (short handle, moulded to the hand, no pomel) to be superior, but the french grip gave me access to certain moves which complimented my style and are impossible with the more powerful but more restrictive pistol grip.
Conversely the pistol grip does suit many people's style and they might find my sword hard to hold on to and lacking power in parries.
The same weapon which is a major advantage in the hands of a proponent of one fighting style is a major liability in the hands of someone whose style it does not suit.
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An additional thing to the answers above mine, a double lightsaber has less control and requires a lot of spinning.
If handled correctly, it is fierce and offensive, and can be hard to defend against.
But the downside of this fighting style is that there are less options/moves to manoeuver with; since you don't wanna hit yourself with the other end (like wallyk said).
With that also comes the fact that when spinning, there are also gaps/openings in your defence that a fast enough opponent can use to his advantage.
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A double-bladed lightsaber (saberstaff) isn't an inherently Sith design, but has been long associated with them.
One of the reasons that not very many Jedi use it is because it is an exotic weapon that only a few can teach (according to The Jedi Path ).
It also depends on the preferred style of Jedi.
Count Dooku, while a Sith, used a saber with a bent hilt to enhance saber to saber combat.
As poepje said, saberstaff maneuvers generally include a lot of spinning, but this doesn't mean there are gaps in the defence.
In Dynasty of Evil , Darth Bane's apprentice, Darth Zannah, uses a saberstaff to defend against Bane's brute force, creating a nearly impervious defense.
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It looks cool, but it's an inherently terrible design for a weapon.
This design, dominated by a long metal hand grip, is impractical in most senses.
One of the first things an attacker would strike (either on purpose, or accident) would be the center of the weapon, causing it to split in two pieces.
This is clearly evidenced by Darth Maul's battle with Obi-Wan Kenobi and Qui-Gon Jinn in episode 1.
What more, in order for the weapon to continue functioning after being severed (as-in the case of Darth Maul), the weapon would need two of everything (two crystals, two power sources, two on/off buttons, etc...), and they'd have to be placed in perfect symmetry in preparation for the weapon to be cut in two and still function.
This built-in redundancy only adds to the complexity of the weapon, without providing any real additional benefits (you basically are designing it to be cut in half, and hoping that when it does happen, it's right down the middle and not off-center, which would render half of the weapon totally useless).
For these reasons, it's far more practical to carry dual light sabers (with traditional standard grip), or use a single light saber as most Jedi do.
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The double bladed lightsaber is a sith design and that is why nearly all jedi reject it's usage, other then as a training tool.
The double bladed lightsaber is referred as a sith design "
Since the time of Exar Kun"
In the book of sith on paqe 85 published in 2013.
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This question always kept coming to my mind. Why don't they use double light saber to their advantage?
In Star Wars, why didn't all the Jedi use the double lightsaber like Darth Maul?
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The double lightsaber is a sith design and that is why nearly all Jdi reject it's usage, other than as a training tool. The double bladed lightsaber is a sith design and that is why nearly all jedi reject it's usage, other then as a training tool.  
The double bladed lightsaber is a sith design and that is why nearly all jedi reject it's usage, other then as a training tool.  