Also, if, for example, cars and airplanes and banks and nuclear submarines would accept executable code as input, some people would do damage on a gargantuan scale.
Clearly, being liberal in what you accept must end somewhere. I argue that it should end very, very soon. Even innocuous things such as "let's allow everybody to read the subject of everyone's mail messages", if available at scale and cheaply, would entice criminal behavior, for example by those mining them for information that you are away from home.
Does anybody know how RMS thinks about passwords nowadays?