Why oh why?

Why do characters in films and on TV always cut their hands? Whenever there is a solemn oath or blood sacrifice to be made, they happily slice into the flesh of their palm, despite knowing full well that they will shortly need full use of this appendage to wield a bow and arrow, wand or suspiciously high-tech ancient vampire killing device. Why not just make the incision somewhere on the arm or leg, where it won't impede your ability to fight?