The smoother the transition to the trap means better scouring of the pipe and fewer deposits or clogs. Also, the longer that dirty arm is, the more chance of a smelly drain. The trap is what stops sewer gasses from coming back up into the house, but the insides of the pipe don't stay pristine...the longer it is, the more crud that can accumulate and start to smell. Only that portion after the trap is blocked from smelling.