You know that clear plastic disc that sits between the cassette and the spokes? Let's say it gets damaged and has to be taken off. Is this something that should be replaced, or is it pretty much optional to have?

I made the mistake of riding a bike with the derailleur out of adjustment, and the chain derailed off the largest sprocket, and tore up that disc. I had to cut out the disc before I could even pull the chain out, it really wedged itself into the plastic. The chain seems fine, and nothing else is damaged.