Cable length has nothing to do with what Kim's talking about. He's talking about the length of the throttle pressure lever. There are at least 2 lengths and it affects the ratio of how much the lever moves vs. the carb. You need a 1 to 1 ratio and the lever that's real long doesn't move nearly as far as the carb pulls it so it doesn't work good. Most cars have the short lever so you may not have seen the long lever.
BTW: I agree that in most cases you don't even need a return spring because the throttle pressure valve inside the trans. has a spring that pushes it back. I did run into 1 that didn't return good on it's own so there are extremely rare cases where it's needed.