>>2020668Derailleur protectors may protect the derailleur but are a total risk factor for your frame.
The derailleur is attached to a piece of metal (hanger) that is designed to snap under exceeding stress levels (like if you smash against a rock), and to be replaceable. That's intended to save the frame. Once you install a derailleur cage, you might save your hanger (not necessarily the derailleur per se), but kill your frame.
Those contraption are only useful if you want to protect something FROM the derailleur. Like you commute on a train\car with your bike and you don't want everybody and everything to be greased up by the derailleur. Or you just want to be able to LEAN your bike on that part.
