Just have to say it...
Mark had a recent piece about a little concept known as "horemetics" which is when small doses of stress (dietary or environmental) might actually have a positive effect on our bodies and overall health. However, the key words here are "small doses."
So yes, eating crappy for a day or two might actually be good for your body by conditioning it to respond to stress better, kick start certain hormones, ect. But the constant stress of a bad diet is going to take its toll over time and wear down your body. We're talking small targeted doses of occasional stress as opposed to the constant, unending stress put upon your body from eating a shitty diet that causes digestive stress and systemic inflammation, ect.
It's all about being reasonable.