For me it depends on the bike, if it's an old Varsity, they don't look right without that huge chrome plate. If your talking about a higher end bike, then I may toss it or just install a small clear one to protect the spokes in the event of an over shift. The make and model of the derailleur also might make me install one too, if your running a derailleur that's known to go into the spokes when in failure mode, then it that pie plate may save your wheel.
Some derailleurs like the Suntour Mountech come to mind in this case.