In my opinion, the two most important facets of paleolithic nutrition are to avoid foods that irritate your gut and to balance your omega-3 to omega-6 fatty acid ratio (aiming for 1:1 to 1:2). Grains have a negative impact on both of these aspects of a paleo diet.
After reading my post on fat, you might have picked up on an important detail about grains. Not only do they contain lectins (like gluten) that damage the cells that line your gut, but they are also very high in omega-6 fatty acids. Grains (including corn) and legumes are high in linoleic acid, the omega-6 fatty acid that seems to be at the root of many modern diseases. Remember that omega-6 fatty acids contribute to pro-inflammatory pathways in your body and that the huge increase in the proportion of our dietary fat that now comes from omega-6s (instead of omega-3s) is a major player in a wide range of diseases.
But it gets worse. These omega-6 fatty acids are concentrated in modern vegetable oils. Oils derived from grains and legumes (soy, canola, safflower, sunflower, peanut, corn, etc) didn’t exist until the process of mechanical extraction was invented. So, not only are you consuming omega-6 fatty acids directly from grain-containing foods, but also from the vegetable oils that they are cooked in.
Another insidious way that grains have negatively impacted human health is with farmed meat. Cows, pigs, sheep, chickens and even some farmed fish are fed grains. The meat from these animals no longer contains a balanced 1:1 ratio of omega-3 to omega-6 fatty acids (which they did prior to agriculture). Instead, it is typically closer to 1:10! It is not enough just to avoid grains in your diet; you need to be mindful of what you eat that eats grains too. In a perfect world, we would all eat pasture-fed beef, free-range poultry, wild-caught fish and wild game meat, while also avoiding all grains, legumes and modern vegetable oils.
When budget becomes an important consideration (like it does for me), I suggest removing sources of omega-6 where affordable, but also focusing more on increasing your intake of omega-3 fatty acids. One inexpensive way that you can do this by eating omega-3 eggs (the ones I buy have 660mg of omega-3 fatty acids per egg and only cost $2.79 per dozen at my local Kroger) or free-range eggs (which typically have a 1:1.5 omega-3 to omega-6 ratio). Another way to increase omega-3 in your diet is to eat more wild-caught fish (canned salmon and sardines are a great inexpensive option). You can also take a fish oil supplement (usually liquid oils are much cheaper than capsules and look for the highest EPA and DHA content for the price). However you choose to increase your omega-3 consumption, omitting grains from your diet is critical. There are no mammals in the wild that have grains as part of their diet. The only group of animals that do well eating grains is birds, which we are not.