The original hard disc is not a tight fit, and allows grease to get where it needs to go.
My take on the necessity of the disc and spring is to reduce wobble of the shaft. A brand new distributor has some clearance or it would not turn. During its useful life (50-60,000 miles when these trucks were built and run on horrible oil) there will be wear causing more shaft clearance to occur, making the spring and disc even more critical for taming the wobble throughout the useful life of the engine.
Carl
