If you're ever more than a mile or so away from your house/vehicle, and you've knocked the rear derailleur out of adjustment, that little plastic guard may be your best friend.... I took it off of my 1st three bikes, but now it stays on my most recent two.... I was riding with a friend about 7 miles away from the parking area, when his chain jumped over the big gear on his cassette, right into the spokes/hub area. Needless to say he broke about 4 spokes before he could get stopped, and had to push his bike over 6 miles back out.... Now ask yourself if it's still "cool" not to have it on your bike..... It does what it was designed for. Nothing more, nothing less.