Well let's start by giving a short description of the persona in question.
Hero: What's a hero? Most people probably have there own definition of this one. A hero comes in many forms: the questing knight, the valiant soldier, and the vigilante are all types of heroes. Basically a hero will not only perform one heroic deed, but he will perform them over and over again without thinking of the reward first.
Antihero: What's an antihero? Well this one is a little more complicated. Just like heroes, antiheroes come in many forms, the dark swordsman, the swashbuckling pirate, and the dashing thief could all be antiheroes. An antihero will do all the things a hero will, save the kidnapped princess, slay the evil dragon, and all those other "heroic" things. However he will use different methods, here is an example, a hero might postpone a battle because an area is crowded with innocent bystanders, but an antihero would just toss a grenade into the crowd and hope he got the bad guy.
Now let's list some reasons why we would want an antihero as the main character over the traditional hero.
Reason #1: A change of pace.
Everyone has played the standard RPG with the standard hero, and to say that the basic formula is old is an understatement. However I am not saying that we should change what works. What I am saying is that having an antihero as the main character would create an entirely new gaming experience.
You would not be confined to the straight and narrow, you could be the "bad guy" while being the "good guy".
Reason #2: nonlinear game play... with ease.
Having trouble with the idea of nonlinear game play because it conflicts with your plot? Like say why would a character who murders and pillages want to save the world? Well that's where an antihero comes in. He could perform the worst atrocities and still want to save the world.
Reason #3: Mercy is for the weak.
Does it ever make you mad when a hero spares a bad guy only to have him come back and cause you countless headaches? An antihero has little need for mercy. The bad guy begging for his life would just anger him. Where a hero might see a villain as redeemable, an anti-hero would just see him as a threat that needs to be eliminated.
Reason #4: More enemies.
By definition an antihero and hero would not get along well, so why not have them fight? It could make things a little interesting if you had to fight both a villain and a hero.
Reason #5: Multiple endings.
Can you image the possible endings for an antihero? He could save the kingdom or destroy it, or maybe he would defeat the villain only to become one himself. There could be dozens of endings for him.
In conclusion let us say that there is definitely a lot of potential in the antihero. Of course there are games that feature antiheroes, but maybe there should be more. That is for you to decide, if you want games with antiheroes ask for them.
Special thanks go to my friend Chris, the perfect example of an antihero.