When I originally started this thread, I felt the Site Lock was just another way to dig a little deeper into the guests pocket. But for the most part, I have since changed my feelings on that.
Many parks like to be able to assign a space when you walk in to register. If they don't have to assign you a specific spot in advance, then they can move their guest around in such a manner as to keep empty sites full, and that's good for the bottom line.
In our case, you book the site you want, for the nights you want, and that's it. By default, you have a price lock. And this is great for the guest, but not so much for the park. Problem is, we end up with orphaned nights when someone books a site for the 1st - 5th, and then someone else books the same site from the 9th - 15th, which leaves the 6th - 8th empty. As we require a 3-night minimum stay, these orphaned sites are now empty and without some special intervention, they can't be booked.
If we were able to move guests around at arrival, we would be able to fill those empty days. At the end of the year, we easily end up with a hundred or more unbooked nights. And we only have 8 sites! Imagine what happens at a park with hundreds of sites!
But each of our sites is unique, and many of our guests are repeat visitors who have a "favorite" site. So while these empty sites are not good for our bottom line, for a park like ours they simply become a cost of doing business.