I just returned from a LBS to have a rear spoke repaired on my Schwinn 700c Hybrid. At the LBS I noticed they did not put back the "pie plate" (as the mechanic called it). It's a round plastic type shield which might protect the rear gears. I mentioned that to him and he said "it will only break and it's not important".
What is the purpose of this "pie plate" and does it really matter if it's there or not?