Chain guards are a, ahem, legal requirement for a roadworthy bike. The main aim is to stop you and your clothing getting caught in the chain. I've seen some photos of a racers foot after he lowsided and got caught up with the chain and sprocket, not pretty in the limping-for-the-rest-of-your-life department.
My own experience is that they won't stop a snapped chain. At three figure speeds on the French autoroute the chain on my ZZR600 snapped. It shattered the plastic chain guard, cut through the plastic side panel, and, luckily for my butt, was stopped by the seat and subframe. In my case the chain bunched, but without damaging the engine cases. Lucky again I guess.
If you analyse where your leg is while riding, I don't think there is much risk of a broken chain striking your leg. But I would have thought a steel/alloy chain guard would be a GoodIdea™.
Iain
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