Actually I have always wondered on the legality of this.(posted 9017 days ago)However in Track & Field, for example, it wouldn't be best simply to bash half the keyboard since (correct me if I'm wrong) if 2 buttons are down simultaneously then they cancel each other out, you'd have to hit whatever keys you assign in a careful rhythmical fashion anyway. I remember someone once telling me about an arcade with an 8- directional joystick used for running, so an idea for people with HotRods is to assign each direction of the joystick to a run button, use your imagination.
But getting back to the question there is also the converse, are you allowed to assign the same key to multiple actions? eg. one key for firing and the same key for bombing as previously mentioned. In Whack'A'Mole type games you could assign one key to hammer all 9 positions at once, and indeed I did this for the game 'Stompin`'. In a platform game I might assign the same key for jumping and climbing etc...
MAME has the facility to redefine the keys in many ways and I always use this to my best advantage, if this isn't allowed it should say so in the rules.