In my experience - if the chain is a quality chain (price usually = quality) and the sprockets are in good condition (new sprockets installed at the same time as a new chain) and the chain is adjusted properly and the suspension is maintained (axle and swing arm bearings serviced regularly, throughly and properly) - a case guard may not be needed. I have never broken a chain, so I can't say, but I have repaired many engines that got intimate with a chain and the cause was poor or no maintenance to the drive line. BTW - I have the OEM "chain guide" on my engine.