My recollection is that at least some of the belt drive models have a rear catcher. Depending on the geometry, that could call for a shroud over the reel, reaching to the back of the mower. If the reel is shrouded sufficiently, there wouldn't be a safey concern over the reel running all the time. (The blades run all the time on a rotary mower, and it isn't all that unsafe so long as you keep all four wheels flat on the ground and keep your feet away from it.)