Sitting on the fence gets you a sharp wooden stake unpleasantly inserted into your rectum.
There are many voices for moderation in diet. I can't tell you how often I've heard and read statements like, "there are no bad foods, only foods we eat too much of," or, "everything in moderation is the key to health," or, "just eat a balanced diet." I was proselytizing the paleo diet to some people I know just a couple of weeks ago over dinner (which I don't recommend, as it both makes you an asshole and rarely makes any difference to the people you're arguing with, but sometimes I can't help it) when one of the people at the table piped up with "just don't overdo anything." Don't get me wrong - the speaker is a great guy - but it's not good advice. And it's not limited to lay people. Dr. Cordain's book made a big point out of recommending we eat strictly Paleo 90% of the time. Mark Sisson argues for paleo eating 80% of the time. Everywhere we turn we hear messages promoting moderation.
Well, mine is not one of those voices. Whenever I hear someone advocate eating a moderate amount of grain (just one piece of pie; just a couple of cookies on your birthday) I compare it to someone smoking just one vial of crack, getting stabbed just once or twice in the heart, or taking just one steel-toed boot to the crotch.
Why do I react so strongly? Let me be clear - wheat has an opiate-like effect on the brain. I'm perfectly willing to admit that this may vary in intensity across individuals, but I am absolutely convinced that it works on me. You can argue with me all day about gluten and how well we have or haven't adapated to it, but the bottom line for me is that wheat is a drug. I never feel better than I do after eating a bunch of cookies or bread or pizza (and no, it's not the carbs or sugar, because I can eat an equivalently large amount of ice cream or french fries or chocolate without anything like the same effect). I feel so good that I instantly want more of it, and I end up eating waaay too much. Yes, enough that I needed to put 3 'a's in waaay.
There is no moderation for me with wheat any more than there is moderation for alcoholics and beer or compulsive gamblers and bets. Eating wheat in small amounts or sticking to a wheat free diet 80% or 90% or any % other than 100% of the time simply doesn't happen. Can I eat rice that way? Actually, yes. Ditto for potatoes. But not wheat.
Am I saying that wheat is similarly problematic for you? Not exactly - I have no idea. Maybe you're not sensitive to carbs, but are prone to alcoholism. Maybe there are no items to which you are prone to addiction. If so, great for you. But I bet that more people, at least Americans, have near compulsive relationships with food than not.
So if you're like me in some way - addiction prone to something or other - don't listen to the voices urging moderation. Go zero tolerance on that problem. Go 80's action movie on it. Declare war. Decide for yourself to never, ever, as long as you live, ever again eat wheat, or sugar, or drink alcohol, or look at kiddie porn... whatever gives you trouble (I hope it's not kiddie porn). Do it. If anyone tells you that it's okay to have just one slice of toast or one sip of beer, kick them in the nether regions with all your might. Or politely decline. It's up to you.
Why do so many authority figures (everyone from doctors to diet book authors to online bloggers) keep advising moderation? Mostly it's a complete lack of balls - balls they don't have or that they don't think you have. They tell you to stick to a diet mostly because they don't think you'd be willing to go 100%. They think you, the reader, would shy away from any recommendation to give up wheat forever, without exception, and instead follow the advice of some more moderate author. They're not willing to tell you the truth - that you probably need to be just as vigilant about grains or sugar as an alcoholic has to be about booze. They're not willing to challenge you. I am (largely because I'm not selling anything, and so have relatively little at stake).
Now there are a few places where moderation actually does make sense. There is no reason to think that anybody gets any benefit from even the smallest amount of grain. But what about alcohol? I'm willing to concede that, for non-alcoholics, moderate amounts of alcohol might be beneficial where larger doses are not. Caloric excess is another example - occasionally overindulging in food is probably good for your body (as is occasionally underindulging). I'll buy moderation in sun exposure as well.
But when you know a food is bad for you, rethink moderation. Look back over your personal history and ask yourself whether giving in to the urge to eat cookies or cake or whatever made it easier or harder to resist at the next meal, or the next day, or the next week. There's a popular psychological model that makes us think that needs or cravings are like steam in a kettle - that you have to "let them out" (i.e. indulge them) every so often, and if you don't the "pressure" will build and build and become unbearable. Does that really match your experience? I know it doesn't match mine. I've been wheat free for over 6 months and diet soda free for 3 months and the hardest periods of time with both were immediately going cold turkey - not months later, after some imaginary build up of pressure. It's easier for me now than it was (though it's still not easy, and any day now I might fall off the wagon).
For the sake of your own long term health, give up on the idea that moderation is a virtue and embrace the cold, harsh reality of a zero tolerance approach to dietary infidelity. Cheat on your diet the way you'd cheat on your spouse - the right amount of cheating is no cheating (at least, the way I hope you'd think of cheating on your spouse). Say no to moderation and say no to getting moderate, average results from your diet and exercise programs.