If you could cum out of your ass, then you would rub your cheeks to masterbate. You would also have more cum come out, and at 10x the speed. If you held in your shit for an entire day, then you could shit and cum would go out at the speed of light, disintegrating anything it touches, people will fear it, they will run from it. But the cum blast will arrive all the same. In addition, you would be able to leave a shit and cum mixture on gas station bathroom floors, which whoever steps in it will slip and crush their skull. So in conclusion, cumming out of your ass is overpowered and could be used to murder several people.