I'm ambivalent about this. 2 of my ducs have open clutch covers and the other 2 don't. I definately prefer the look; watching the clutch spin is mad cool! As for the rattle....well...sometimes i like it and sometimes I just find it annoying. It depends on my mood.
The other day I was at the parts counter at martin eurosports. The guy standing next to me was working w/ Jeff the parts guy to order an open/vented clutch cover for his new 1098. He asked Jeff, "Can I order a vented clutch cover so dirt and water can get into my clutch?" I cracked up. He was being a wise-ass, but his point is well-taken. Ducatisti run open/vented clutch covers because it looks/sounds cool and makes the bike more like a race/track bike. Of course, in the context of racing, the benefits, (cooling and reduced weight), outwiegh the detriments, (accelerated wear). In your typical motogp race the clutch only has to last 45 minutes, so accelerated wear is never an issue.
From a strictly logical perspective, running an open/vented clutch cover on a street bike is idiotic. But many of us, (myself and Mr. WiseAss included), do it anyway. Life is not logical.