Originally Posted by
Velo Dog
Never used one, but do you NEED one if the bike's set up properly? I've only had the chain slip off very occasionally (like maybe four to six times in 30+ years of riding), and it's always been because I'd been sloppy in the derailleur adjustment.
No, you don't really
need one. But it can be good insurance against the occasional slip up. I put mine on when I got a custom paint job. I didn't want to risk it.
On a regular double setup, you would rarely need it. On a triple, the chance of the chain falling off the inner ring is a little greater. On a compact double, it is greater still. On a single speed <edit>(or 1 X 7,8,9,whatever)<edit>, especially one used offroad, it can be pretty useful.