I read somewhere once that the reason for the kickdown return spring on a kickdown cable is to help make sure the lever returns when you let off the gas. If it hangs up your carb can get stuck at WOT just because the throttle return spring isn't strong enough to overcome a sticky kickdown lever. This was the reason the factory kickdown linkage had the long slot in it - to allow the carb to return to idle if the lever stuck. Probably a rare, rare occurrence, but the spring sounds like cheap insurance to me.