Mine broke at a very young age when I threw a chain doing a burn out in my younger days, when I convered my Commando to the Featherbed frame and rebuilt the motor for it I grounded that off completely, I run my oil tank and crank breather hose into a large catch bottle in between my engine mounts and I only lube my chain when needed, I don't get much crap spraying onto my motor at all and its been like that for over 37 years now, it also makes it easier to check your front sprocket or putting a new chain on without that cover part.
A few years ago my chain threw a retaining clip and my chain got jammed up between the inner case , it was easy to unjam it but if the cover was still there it would have been a major job to free it up on the side of the road and the funny thing was the spare retaining clip I had on my clutch cable thats been there for over 40 years was the wrong size clip for my Norton chain, lucky I wasn't to far from home lol.
Ashley