I, like many others, bought a MH for the convenience of moving around, making lunch, using the restroom etc while going down the road. That said, while you are sitting down either on the couch or dinette, you buckle in. I mean why not?
Sometimes DW and son will watch a movie in the bedroom while laying on the bed. No belts for this but I imagine the prone position provides more surface area to mitigate sliding or being thrown in a hard stop. Not saying I reccomend this, but it's how we roll.