I, too, was able to hike 20-35 mile days on the PCT on nothing but absolute crap for months on end. However, the ultimate result was that my metabolism was destroyed. I'm slowly clawing my way out of that two years later and let me tell you, it is a painful struggle. Eating healthy food is not a problem, it's how truly awful I have felt the last couple of months.
I don't think a little junk food here and there when backpacking is going to hurt you, but months on end of it may do a lot of harm, at least for some of us. It may be worth it to hedge your bets and opt for healthier versions of your favorites, if that is at all possible for you.
