Leaving the spring in play reduces vibration in the whole throttle setup. No matter how little slop there is in the linkage and the servos thew whole mess can vibrate. The spring, by keeping a gentle load on the linkage will reduce the vibration. This is especially important for those who feel they need metal gears in their throttle servos; as they tend to have more slop to begin with and wear much faster than composite gears.
Pete