I think the disc and spring are to maintain constant point gap rather than rotor to cap clearance.
I have not seen a broken cap or rotor as a result of the disc and spring missing.
With a distributor missing the spring you can clearly see the points gap change while shacking the shaft.
I had a 216 in my shop that had a noticeable miss on two cylinders at idle.
The customer had tried everything he could think of to fix this.
I only needed to open the hood for a few minutes to to see the problem. His distributor had a grease fitting where the grease cup should be. I had seen this before and it is quite common on a rebuilt distributor. It is also common for those rebuilders to leave the disc and spring out.
Once I put the disc and spring in the engine miss went away.
