Paleo's main problem with grain is gluten and the response the body has to it, treating it pretty much like it would a virus. Then, there's lectins which literally scour the gut of probiotic bacteria, reducing your ability to digest. Beyond that, phytates, which bind micronutrients - it is true that there are all manner of micronutrients in grain, but you won't get to ingest any of them.
You only need to come into contact with grain once every fortnight to maintain that level of damage. You've become tolerant of the pain and damage.