many games have the option to change it to numpad for people who use the mouse in their left hand.
I'm left handed and I use the mouse with my left hand, and I use wasd keys because it gives me more options (and I'm fairly used to it by now). Arrow keys can be a problem for laptop users(such as myself) because there is no numpad. Today PC games tend to lean towards wasd keys, and with good reason. It's simpler, easier, and you'll have a consistent feel for the game.
i think wasd is an option for left handers or injured people
Where did you get that idea? a lot of right and left handed people use wasd because of simplicity and the combination of options it gives. Arrow keys are for simple games like...platformers. A good example of a game where arrow keys fit would be "Give up", "One level" and "exit path".
arrows are supiorer because they are awesome
Same could be said about wasd keys. There are actually more people who use wasd keys than there are people using arrow keys, I personally feel more comfortable with the wasd keys, I'll stick with what I know best.