I think grain free is a fad. They replace grains with peas and potatoes. Still a carb, still a starch. Still a food that has to be processed in order for a dog to break it down.
So, really what is the difference?
I avoid corn because most corn is contaminated with GM corn.
I have two dogs on kibble, not grain free, and are doing just fine.
I have two dogs on RAW. Occasional carbs from fresh fruits and veges.