I do exactly that. One day a week. I don't usually have dessert though, but only because I don't seem to like sweets as much as a lot of people. It's not every week, but I allow it every week. I allow myself 3 oz of pasta cooked (actually a more decent portion than it looks like in writing), or a po' boy which means a fairly large roll.
If you don't gorge on the one day, you're actually at 86/14. If however, you eat twice as much on your non-primal day and it's all non-primal food, you could easily go over 80/20. So indulge. At 80/20 most of us are still better off than we were, and we're a damn sight better off than people still eating out of boxes, the frozen prepared food section at the grocery store, and chain restaurants.
As a note, I don't have much of a reaction to non-primal foods. Worst case scenario is a little windiness. I'm not sure I'd do it if the next day was going to be painful.
"Right is right, even if no one is doing it; wrong is wrong, even if everyone is doing it." - St. Augustine