I'm actually surprised that no one has touched on the legend of why everyone wants the spear, and some of its legendary owners...
Much of the mythology surround the Spear of Destiny revolves around the Indiana Jones like ideal that it can be used at the head of any army to lead them to victory, and that losing it is the sign that divine favor has left you...
Barbarossa won many a battle, until he drowned in a river after falling from his horse, spear passed on, HRE slowly declined....
Hitler had it, Then Americans came into Berlin, taking the artifact, Germany falls....
Etc, etc, etc. And that's if you believe much of the conspiracy theories surrounding it.
It's like the scabbard of Excalibur being more powerful than the sword (granted immortality). The sight of the spear is a sign of divine favor and makes an army undefeatable. Imagine fantasy kings fighting to recover that kind of artifact...
As for Longinus, in reality he did become a martyred saint, stoned to death as reported earlier. Roman soldiers used to roam battlefields after battles, striking the heart with their spear in order to end the life of those who were injured on the field of battle. There is conflicting information as to whether this was done out of mercy or necessity though. His immortality would be a confusion with the Wandering Jew story (which also has about a dozen different versions). But as this is a curse laid down by Jesus (a supposedly very nice guy by most accounts), the story seems out of place.
BTW--The spear in the People's History (or Heritage) Museum in Austria (Used to be Hapbsburg Museum) is the one that is referred to as disproven. You should be able to find a picture of it online without much trouble.
Best of luck!
direheroics