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?
His mistake in taking it off. He was lazy or forgot. I believe its purpose is to keep the chain from getting jammed between the frewheel and the spokes should the derailer fall out of adjustment. It is there for a reason, so ask him ro put it back on and to ask you before making any decisions about your bike. Don't tip him.
I called the LBS to tell them to hold my pie chart as I would pick it up tomorrow. They confessed that "it cracked" when they were removing it to repair the spoke and again tried to explain that when the gears are properly adjusted it's really not needed. I'll just forgot about it and not pursue the issue any further. Maybe I'll also look for another LBS.
Your question about the pie plate has even been topics in bicycle magazines. Most
people think there useless, and look stupid. Then some insist you must have it, just
keep it adjusted correctly, you will probably be fine. They should have been up
front with you from the beginning though, I would look for another bike shop.
My new bike, a Globe about one month ago got the chain wedged between the pie plate and the gear next to it. I had to stop, get off the bike and pry it out from between them. I was shifting to the lowest gear at the time. Iam sure it is a adjustment thing that my LBS can adjust when I go in for my first check up. Without that "dorky" pie plate I wonder how much damage would have been done??