I agree that, when you are seriously hiking most of the day that a high fat diet is not a problem, and you really only need enough protein to allow normal muscle tissue maintenance, plus a bit of muscle growth. Most Americans eat far more protein than they strictly require for health.

For me, the biggest reason not to substitute protein powder drinks for some or all meals, and the most overlooked part of a backpacker's diet, is fiber. At home I eat a lot of vegetables and whole grains. If I suddenly switch to a trail diet with almost no fiber, it plays holy heck with my digestive system. Protein drinks, for obvious reasons, often have negligible fiber or no dietary fiber at all. Nuts and dried fruit are two excellent trail foods, because they are not just concentrated, but also are unprocessed real food.