Plastic "pie dish"

[lunacycles]

it's true. the "pie plate" is one of many, many things bike snobs use to identify a newbie. It's totally unnecessary if your bike is set up correctly, but as a poster noted, it might save your life some day...

I routinely remove them, and never install them on any of the bikes I sell. That being said, who cares? If it doesn't bother you, it might be your friend some day. 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.

the one thing I've always disliked about them is that uv exposure turns them brittle, their little clips onto the spokes break as a result, and they become a rattling nuisance. But, otherwise, they are harmless...and weigh approximately 42 grams.

[TxDoc]

It's totally unnecessary if your bike is set up correctly

Exactly.
And, if you are going a little bit faster you will notice the horrible noise they make...

[laura*]

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.

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.