I am not totally convinced that they are for structural safety so much as exterior ridgidity for the knuckle head bus drivers. My bus has one at the very bottom of the skirt. I can't see how this would provide safety to the passengers except maybe to prevent another vehicle from running under the bus in an accident. I see the placements of the rails more in the fashion bump guards are installed on passenger vehicles. They are designed to prevent more extensive damage to the rest of the body in the event of a small fender bender.
I will be taking the two middle rails off of my bus and leaving the top and bottom rail simply for cosmetics. I have already taken a few pieces off and I think it does look better. Not quite so "industrial" you might say. However, my wife said she likes it better with them on. Opinions vary i'd say. I am basing our conversion by asking myself this. "Do i want a RV that looks like a school bus, or do I want a school bus that looks like an RV?" I choose the latter and that is why I will be investing as much time on the outside of our bus as the inside of it. We've spent over $700 on the outside of the bus trying to make it NOT look like a school bus, of coarse i'm going to remove stuff like rub rails and warning lights.