The advantage of not mixing, from a passenger POV is that you'd know what you're getting. As a solo, I don't need the space of a bedroom on the Florida trains. I would be happy in a roomette if I knew it would be a VI and I'd have my private toilet and not have to walk down the hall to a bathroom. But when I don't know which car they'll give me, I book a bedroom and try not to think about the cost.