Originally Posted by
deep_sky
Even if your frame is metal, the chain can dig deep gouges in the bottom bracket shell/chainstays which is a)unsightly and b) can be a stress point for future failure of the metal...no reason not to have one no matter what your frame is made of.
EDIT: The chaincatcher protects the frame by catching the chain if it is thrown inside the smallest chainring and guiding it back to the ring. Sometimes even a perfectly adjusted drivetrain throws the chain and then it can jam, causing gouges like what the OP experienced. There are different styles of chaincatchers, for both clamp-on and braze-on derailleurs.
Yep, I have chaincatchers on both my aluminum mountain and carbon road bikes. I absolutely recommend a chaincatcher for all carbon frames, as I once saw a guy damage his expensive road frame around the BB area from dropping a chain. A $10 chaincatcher could have prevented all that heartache.
And a chain can drop for a myriad of reasons, not necessarily from slack. I have dropped a chain from hitting an unexpected jarring pothole before.
Chaincatcher is a no-brainer, folks!