I've run open belts for the last two years, right up until very recently. I had the SpeedyMoto covers on there and although looking cool, I had a close call. I was riding home and went over a bit of gravel. A few small rocks got kicked up somehow and started bouncing off my engine. I just happened to look down (dumb, I know) and I saw a rock bouncing around the horizontal belt. I immediately pulled over as I had a feeling something bad was about to happen...
Luckily nothing happened and I proceeded to get rid of the offending stone. I rode home and quickly replaced the open covers with closed ones.
Last thing I need is some engine damage.
Paranoid? Maybe. I never had any issues with the open covers but that event changed my mind, even if it was a non-event.