Why do Jedi use lightsabers instead of blasters? Blasters seem to be more practical. Is there a special bond or something between lightsabers and Jedi?
Obi Wan explains this in the novelization of Star Wars: Episode IV - A New Hope:
"This was the formal weapon of a Jedi Knight. Not as clumsy or random as a blaster. More skill than simple sight was required for its use. An elegant weapon. It was a symbol as well. Anyone can use a blaster or a fusioncutter — but to use a lightsaber well was a mark of someone a cut above the ordinary."
― Obi Wan Kenobi (quoted on Wookieepedia)
Even the official Star Wars website says that the Jedi and their lightsabers were inextricably intertwined:
Although use of the lightsaber is the mark of a Jedi, it is also used by their sworn enemies, the Sith. Typically, a Jedi’s lightsaber blade is colored green or blue. Other colors are rare but possible, most notably seen with Jedi Master Mace Windu’s purple blade. Sith uniformly use red-bladed lightsabers -- an intense, aggressive color that represents their view of the Force.
An essential rite of passage for young Jedi is the Gathering. In the Gathering, younglings go to the frigid ice world Ilum, where they search for kyber crystals to build their first lightsabers. The crystals are located in the planet’s Crystal Cave, an intimidating maze, and “call out” to the Jedi they are in tune with. Once the correct crystal has been found, younglings rendezvous with Huyang, and ancient droid who chooses the parts necessary to build a lightsaber. With everything in place, younglings complete the ritual by constructing their Jedi weapons through a graceful use of the Force.
One might think it's because Jedi/Sith can deflect blaster bolts with a lightsaber or their hands.
INT. CLOUD CITY - DINING ROOM
The mighty doors to the dining room slide open and the group enters the dining room. At the far end of a huge banquet table sits Darth Vader. Standing at his side and slightly behind him is Boba Fett, the bounty hunter.
Faster than the wink of an eye, Han draws his blaster and pops off a couple of shots directly at Vader. The Dark Lord quickly raises his hand, deflecting the bolts into one of the side walls, where they explode harmlessly. Just as quickly, Han's weapon zips into Vader's hand.
But that was just Vader using a Crushgaunt. (...which can apparently be used to grab and hold lightsaber blades. Yikes.)
However, in I, Jedi, we find that Luke believes Vader used Force Absorb on Han's blaster fire instead of Force Deflection. In the EU, Corran Horn was known to absorb blasts with the Force, so this belief is not entirely unwarranted.
Nevertheless, these options suggest that firing blasters at Jedi/Sith isn't always profitable.
Jedi are protectors and defenders, and the Way of the Jedi is the way of peace. While a gun is an effective weapon for defense, it really shines on the attack.
A lightsaber, on the other hand, is a perfect defensive weapon and a moderate-to-poor assault weapon. If you are ten meters from a Jedi armed with a lightsaber, you are relatively safe from attack...unless you start shooting. A lightsaber is literally incapable of firing the first shot, but just as literally able to return fire. If you get hit by a blaster bolt while fighting a Jedi, it probably came from your own blaster.
This actually makes the lightsaber a weapon of humility, limiting the sorts of things a Jedi can do. A mugger or a cop can completely command you from 20-30 meters, and a sniper can deny you access to an entire open area.
Finally, the lightsaber makes it easy to disengage from a standoff. If a shooter and a Jedi are in a standoff situation, either one can back up with both weapons trained until the shooter feels safe enough to put down the blaster. The Jedi has limited options for attacking a non-firing shooter at range: either throwing the lightsaber (which leaves the Jedi open for a blaster counterattack) or using Force abilities unrelated to the lightsaber. Once the shooter holsters the blaster, the Jedi can safely disengage their own weapon. As we see in the real world and/or modern-day stories, a gun vs. gun standoff is much harder to negotiate: the shooter who puts the gun down first is at a tremendous disadvantage.
All in all, using a lightsaber instead of a blaster makes a Jedi a low-level threat before you start a fight, and a high-level threat during the fight. This helps defuse situations before they start, convincing would-be assailants that they will live a little longer if they give peace a chance.
Because guns are crude!
In Episode III: Revenge of the Sith after defeating Grievous with a blaster, Obi Wan that blasters are 'So uncivilised'.
Wookieepedia explains that these weapons were:
Designed as much for elegance in combat as for ceremony
If we look at the origins of the lightsaber, they started out as forcesabers through which:
Users [could] channel the dark side of the Force through black laboratory-grown crystals which would create the glowing energy blades
There is also a great symbolic and functional value attached with a lightsaber:
The lightsaber was considered more a tool than a weapon. The lightsaber came to fulfill both a symbolic and pragmatic role for the Jedi; it identified the wielder, had a powerful visual impact, and given the availability of cybernetic replacement limbs might have been seen as a more 'clean' weapon than the blaster, which Obi-Wan regarded as "uncivilized," "clumsy" and "random."
The lightsaber is a weapon inextricably bound with the Jedi Order and its history/mythology, designed for elegance in its use in both combat and ceremony, an attribute lacking in 'blasters', which is constantly stated by Obi-Wan Kenobi who is renowned for his contempt of the 'crude' weapons.
You are correct in your assessment of the lightsaber as being a weapon of limited strategic value, and it is for this reason that modern-day military forces make little use of melee weaponry. For this reason, many separate types and classes exist including the Jedi Sniper, who excel at using the force to enhance their ranged accuracy with Lightsabers.
In addition to the Jedi Sniper, it is alluded to on the Jedi Wiki that the Jedi "Weapon Master" can make use of ranged weapons as well as the melee-based lightsaber:
"... double-bladed lightsaber, lightsaber pike, quarterstaff, San-Ni staff, whips, or flails, was all that needed to be done to be considered a Weapon Master by the Jedi High Council. Additionally, ranged weapons and the use of ones body as a weapon are sufficient to receive this recognition." source: http://starwars.wikia.com/wiki/Jedi_Weapon_Master
Jedi overcome any disadvantage associated with their melee-based combat by perfecting the ability to deflect projectiles with the Lightsaber, a phenomenon commonly exhibited within all Star Wars movies, the use of the force to lift and propel objects as a form of ranged combat and through the aforementioned limited use of ranged weapons by specialists. It may also be noted that Jedi do occasionally use ranged weapons should the situation call for it, such as the death of General Grievous via blaster at the hands of Obi-Wan and a convenient nearby blaster in episode 3.
In short, the ranged disadvantage associated with lightsaber combat is greatly reduced through deflections, the use of the force and the actual 'throwing' (for lack of a better term) of lightsabers as a form of ranged Combat. The Jedi Order's persistence in using the lightsaber is due to its historic and symbolic value with the order and its mythology, and has become symbolic of the order itself and its mission to uphold galactic peace.
A similar question can be found here: Why Do Jedi Use One Lightsaber in Combat? It elaborates on the use of the Lightsaber as a primary Weapon.
Hope this helps.
Just an observation (as a science fiction author myself): almost all of the reasons given above (and indeed all of the quotes from the Star Wars Cannon) are taken--often verbatim--from the complaints of "gentlemen," for the last thousand years, bemoaning the fact that firearms allow "the unwashed" to defeat a "gentleman" in combat. It takes "patient, loving practice" (to quote Heinlein in Glory Road) to gain skill with a sword, and only the upper classes had the leisure time to gain such skill, so that any upperclass person, walking around with a sword on his hip, was the superior of any number of lowborn footpads armed with rocks and sticks. Firearms are indeed "inelegant," and other such disparaging terms... but they WORK. Colt "made all men equal"... and the upperclass doesn't like that very much. Technology has been doing this for thousands of years: making a skill which took much time to learn obsolete... and then receiving many complaints from those who had taken the time to learn the skill in the first place. Lucas simply tapped into a common attitude of complaint on the "swords versus pistols" debate.