There's hardly any calories in vegetable juice. You can always add leafy greens with bananas and make green smoothies if you're worried about not getting enough greens, but usually people aren't getting enough fruit because fruit has been vilified.
If you're underweight and you're health is starting to fall apart, most likely you're not getting enough calories, so definitely smoothies to help you get the carb calories in.
If you're overweight and trying to lose weight, you're eating too much fat and protein calories. Carb up, but keep your fat & protein intake low.