I do think the nutritional benefits gathered from green smoothies (especially those consisting of kale) out weight the cost of food combining. Since I've been drinking green smoothies I am rarely ever sick (haven't been sick in 3 years) and have never experienced greater vitality. The leafy greens and fruit i combine in my smoothies digest like liquid sunshine. I feel amazing immediately, bursting with energy. This is also true when drink a large cup of wheatgrass (4-5 ounces) on an empty stomach.
Everything in life seems to have a trade off. When it comes to weighting the nutritional benefits of consuming large amounts of raw dark green vegetables like kale at the cost of blending it with a little fruit, i personally find it well worth it :)