The intent of the permit is to spread out the use of the trail. This means that hikers should only start at Campo on the day of their permit. Can you get away with starting on a different day? Probably. But wouldn't you want to do the honorable thing and help protect the PCT trail experience for current and future hikers?
YES. As a veteran of many long AT sections, including a flip-flop thru-hike, I've seen a dramatic change in the AT hiking experience due to the increased popularity.