I think people overload their green smoothies with fruit.
You can replace the good consistency of bananas with avocado and then use berries as a sweetener. I then like to cram as much kale, spinach, celery as possible in it.
Of course, maybe you're just referencing fruit smoothies. Those are definitely a once in a while treat for me. I drink 1-2 green smoothies per day. Heavy on the greens, light on the fruit. I like using unsweetened coconut milk as the blending medium.