LOL - so many people here are strongly debating scenarios they're inventing for the purpose of their point of view. Which can be the right approach if you're truly answering the question of "would you?"
The OP has repeatedly pointed out the CG where he does this is likely to be quite empty when he camps - he says he only reserves the 2nd spot if when he calls on Wednesday the park is more than 50% empty.
What any campground (or even hotel) is offering is a perishable resource. 1 spot/room for 1 specific night. If they don't rent the space for a night, it is permanently unsold. In a commercial setting (hotels), they base their fees/rates on an occupancy rate average and look at their $$$ from a quarter/year perspective for the entire facility. I used to manage a with similar perishable offerings and it was a PITA.
In a state park, they use very different metrics to set their prices. So if the OP is paying for a second spot that would very, very, very likely go unsold otherwise he is actually helping to minimize future price increases (or worse yet, park closures).