Yes they are good for many types of camping and you are better off in bad weather to have a solid roof rather than having something designed for good weather.
Depends on what type of items you intend to haul in the trailer. If your hauling things you don't want exposed to the elements then yes it would be worth buying. But if your hauling things like scaffolding no because you wont the room to haul the braces and planks too.
Roof trailers are worth buying for the right person. it depends on what you are using it for. Roof trailers are great for extra storage space and for moving.
I feel roof over trailers are worth buying for two persons. If internet facility and phone is there with us ,buying roof over trailers are worthy than any others. It would be very exciting also.