I am not one to tempt fate, but my RS Turbo runs without a belt cover. Admitedly this is in part to adjust the cam timing in addition to the belt covers requiring ingenuity to fit with half of my engine now being Zetec based.
Over the past 3 years it has been fine over 3,000 miles. That is not to say that I wince at the belt coming off mind you. Many people with ZVH engines (Zetec bottom end with a CVH bottom end) run without covers. I actually have some Zetec and CVH covers to try and make fit; I may just get away with making the CVH items fit.
However, I will be changing the belt this year, a year before it is due and well before the mileage. Why? Due to the previous breather setup the car breathed oil mist in the O/S corner of the engine bay, right where the cambelt is. I do wonder whether with the belt cover it may have survived better.
I could fit the top half of the cover on ; that bolts on easily. The bottom half is where all of the troubles are.
OTOH, many cars run without covers. Many Lotus twin cams ran without belt covers from factory and my Clio also comes into that list (sort of). On mine if the auxilary belt snaps there is a good chance that it will take out the cambelt due to the belt having nowhere else go thanks to the chassis leg sitting next to it and the cambelt cover not quite covering all of the belt:
Yes, the belt is ever so slightly exposed by the crankshaft pulley, and the intervals on that engine are 5 years or 72k...
That said, I would rather run with a cover if possible.