I've never camped without reservations. I'd much rather risk losing a nights site fee than risk getting to a campground and having to turn around and go home after packing everything.
You may be able to camp during the week when it is not that hard to find spots, but with work, and kids in school, weekends and vacations are all we get. Even reservations are hard to get on weekends around here. Weekends at the beach, or lake during the summer are about impossible unless you reserve a month or more ahead of time.