Books could be written.....
The cynic in me believes that FP+ is a subtle way to ensure that guests who come to Central Florida for 7 or 8 days use all of those days (or maybe all but one of those days) to go to WDW instead of 4 days at WDW, two days at US and a day at SW. By way of example, in the past, (and I admit that I am making up numbers here), 95% of all Epcot guests who met the height requirements and who did not shy away from thrill rides, experienced both Soarin' and Test Track in the same day. By tiering those attractions, they have made it more likely than before that people will visit Epcot over the course of two days and get a FP for one attraction on one day, and a FP for the other attraction on the second day. Whereas people might have squeezed all of Epcot into a single day before, it has become very hard to do that now unless you are a RD person. Arrive at 9:30 with a FP for one of those attractions and you are all but assured of waiting in a 70 minute line for the other. Many people will do that. But many others will opt for the FP strategy and do Epcot twice instead of once. The FP+ system has made it such that unless you are a dawn 'til midnight commando, it will take you 6+ days to do and see everything you want to at all four parks unless you go when the crowd levels are 1-4. I do not think that it can be debated that Disney does not want you spending four days at WDW and three days elsewhere. They want you for six or even seven days. I think that they have devised a strategy to make it more likely that you will a) stay on site, and b) spend most of your time at their parks and restaurants. Just a theory.