I use them to add bulk without affecting the flavour. I recommend making your smoothies with at least four ingredients, preferably more, of which at least two of which are strong. The two strong flavours provide contrast with each other, and the rest fill in and bind the taste together. Bananas and yogurt do very well to bind the smoothie together. If you just put a bunch of fruit in a blender, and just use water to keep it liquid, it will taste just like that - water plus fruit.
Natural sweetness is far easier and better tasting.
If your smoothies aren't sweet enough, use honey. Almond milk, almonds, bran, buttermilk, coconut milk, flaxseed and flaxseed oil, ginger, honey, maple syrup, milk, mint, peanut butter, pumpkin seeds, red wine, rice milk, soft silken tofu, soy milk, sunflower seeds, tahini, tea, vanilla extract, yogurt, protein powder.