If you want to stop the bike when the single carb return spring breaks, keep the "B cable". The pull ("A") cable doesn't really push the slides down well, and springs do break. Any piece of metal wire that is constantly bent will eventually break from metal fatigue. The timing of such a break is usually when least desirable.
The early 750 and many other carbs, has a spring on each carb slide to close them. If a single spring broke, the engine would still be controllable. So, the single actuation cable they have, does not factor into positive slide closure.
When the 4 carb set was converted to a common shaft actuation arm arrangement, 4 individual slide springs were removed and replaced with a single common return spring. A single point of failure can make the engine uncontrollable, so a positive return cable was added. The brakes alone aren't going to stop a screaming 750 motor. If the return spring breaks while in an inside turn with oncoming traffic...splat. (Just think about all the times where cutting engine power was essential for survival.)
If you remove the ""B" cable, then for safety sake add the return spring replacement as a routine replacement item, say every 5 years (A guess... I don't really know the MTBF of a throttle spring) and be certain to NOT nick the wire anywhere along its length to instigate a metal fatigue fracture point. You don't really want to gamble that the spring will last forever (it definately won't) and replacement should be at a time of your choosing, not the random point where the metal fatigues to the point of failure.
You can use the "B" cable as an "A" cable for emergency purposes on a CB500/550 with the early non-PD carbs. The throttle will work backwards, but you can control the engine speed.
Ride safe!!
