I've been making green smoothies for a while, but was recently reading about adding protein powder, in particular whey protein powder. Do I need this if I often make homemade kefir based smoothies? In general I usually just make them with yogurt, kefir, or milk, fresh or frozenl fruit and raw greens. I usually steer clear of powders or supplements unless it's powdered spirulina or flax or something like that.

What's your take on the protein powders? Do I need it? 

My personal take on protein powders is that you shouldn't need them.

If you eat a good amount of whole foods including greens and put in high protein sources such as chia seed, spirulina, flax, etc... then you are likely getting enough protein. The dairy products you add as well boost the protein content. I steer clear of supplements as well and do my best to eat only whole foods.

I agree... steer clear of any powders that are not whole foods - there are plenty of whole food vegan protein sources, greens included