Well, as Unkadug explained, they really don't gain. They actually lose. Parkhopper costs what now, 55 bucks if I remember correctly? So, either you buy the parkhopper for $55 (or $45, or whatever it is), or you use one of those extra days for which you only paid a few bucks. The difference between a 9 day pass and a 10 day pass is less than 3 dollars. So, in your scenario, for less than three bucks, you can enter two parks in one day if you don't mind using up 2 park "days" at once. So by doing that, rather than purchasing parkhopper, Disney is losing around $52, give or take some change.