A small point on the computer as a compositional tool. I mainly compose on the guitar, but that is because of the immediate feedback of the instrument. When composing (for example) in FL Studio using the piano roll, pressing the space bar can give you immediate feedback of what you are doing in the same way.
All of the previous points regarding knowing what you want to do over the entire piece of music are legitimate, but being able to steer the music based on what you are hearing at the moment has validity as well. I think this is becoming a point of compositional style, not piano vs. PC as it were. Let's bring this down to pragmatism: if it works, use it. If what you are doing doesn't work, whether it is sheet music or piano pounding, try something else! 'nuff said.