Originally posted by MarkJerling It serves no useful purpose at all. It supposedly protects the spokes should the chain come off the cassette, but it the chain is set correctly, that never happens. Put another way, a more useful device is called a "chain catcher" which can protect your frame and stops the chain from getting mangled if your chain comes off the smaller chain ring, a much more common occurrence.
See
Eurobike 2011: The rise and rise of the chain catcher | road.cc I think protecting the rear derailleur (and hanger) from the spokes is a primary reason for them too!
I've seen what happens if the derailleur goes into the spokes while riding and it's not pretty! It happened to a buddy of mine while we were mountain biking, miles from anywhere. We ended up shortening the chain and doing a single speed conversion in the field to get him back home! The spokes were ok but the derailleur was bent and the hanger snapped right off.
I still remove them too. I like as little stuff on the bike as is necessary.
---------- Post added 02-08-18 at 02:21 PM ----------
I used to have a
Third Eye Chain Watcher on my old 3x8 mountain bike. I was plagued with chain drops and chain suck and this cheap little piece of plastic fixed it!
I no longer need one on my 1x11 rig.