OK, it's not to keep the RD out of the spokes, it's to keep the chain from tearing up the spokes behind the gear cluster.......

'Uncle Knobby' from Mountain Bike Magazine, years ago, called them 'big wedgies', cuz some noob wrote the mag asking about them. He said, 'go to your shop, ask for a big wedgie, they'll set you right up, they always have plenty on hand'. My bud & I have called them that ever since.

Never mangled the RD; broke a few spokes, but never with the drivetrain maladjustment. Sooner put a Wal-Mart bell on my h-bar than run that plastic *** on my bike!

My mountain bike from 1996 still had it last summer when I sold it. IMO, it's there for a reason. Leave it.

Of course if you ride a single speed, fixed gear or IGH bike there is no need for one and they are not fitted.

Another CPSC mandated safety device for bikes that is supposed to make them more idiot proof, like all of the mandated reflectors. Probably very worthwhile for usually improperly adjusted even when new big box store bikes but serves no purpose if the rear derailleur limit screws are properly adjusted. That is until the bike falls over and the derailleur hanger or derailleur gets bent.

Well, the chrome steel ones were...

I had one on my old bike. I stopped riding that bike for a few months, and then took it out for a ride when I decided to sell it. In testing out all the gears, I ended up sending the RD into the spokes like a kimura arm lock.

Here's a pic where you can clearly see the good-for-nothing noob ring that failed to protect my spokes or my RD.

My Jamis came with the noob ring and my felt did not. I wonder why?