In practice I have gotten away with a broken cable on the return pull but that doesn't mean it is a good idea, as I recall I got home from work ok but did feel the need to replace it, and I will put up with a fault that most would not, before now I have ridden my cb500 home with no throttle cables at all, by just winding the tickover screw to the stop and maxing the choke, a ride that was quite unpleasant to say the least. At the end of the day that return cable is there to stop the carb sticking open, and Honda presumably thought that it might.