The main problem that I see with leaving the cover off is the potential safety issue of sticking your fingers or other body parts into the belt when the engine is running. I know, you shouldn't stick your fingers into any thing moving (or a lot of things that aren't moving), but accidents can happen.

95 Contour Zetec, ATX (wrecked, sitting in the barn) 95 Mystique Zetec, MTX 2000 Taurus SEL 3.0L Duratec 1994 Crown Victoria LX 4.6 V8 1993 Chevy Silverado 4.3 V6