Quote Originally Posted by lunacycles View Post
Also, if your chain does fall behind your cassette, in addition to potentially seizing the rear wheel it can also really dig into the spokes and compromise/make them weaker as a result. I have seen a lot of drive-side spokes frighteningly compromised from this occurring.
At the bike co-op where I volunteer, it is a regular occurrence for people to come in with a broken drive side spoke - and when we get the cassette/freewheel removed, all the outer drive side spokes are mangled.

Quote Originally Posted by lunacycles View Post
It's totally unnecessary if your bike is set up correctly
Bikes fall over, derailleur hangers bend; limit screws might be set wrong (because the shifter is set right); twigs can snag in derailleur cables thus changing gears; twigs can snag in chains or a chain can seize, thus causing the derailleur to get pulled into the cogs; and so on. In other words, even when a bike is set up correctly, things happen. It is much better to have just a destroyed derailleur, than a destroyed derailleur and rear wheel.