If you need to buy a car to have as a toad it certainly would be cheaper to rent. But it's not convenient by any means, not to mention the availability of a rental place near your destination. What happens if the MH breaks down, you are completely stranded. I'd hate to have that on a lonely highway, especially at night.
Most people should have a car that can be put on a dolly. That would be the next best thing if you can't or don't want to or don't have a car that can be towed 4 down. For 5 or 6 car rentals costs your going to be close to purchasing a dolly anyway, so that would be my choice.
To rent a car every time ... Not convenient, no safety factor if stranded, nothing gained for your money, restrictions, not as comfortable as your own car, pray you don't damage it, etc.