My take on FP+ is this. WDW is giving you an opportunity to walk up to three, maybe more rides of your choosing and basically walk on for free ( not counting cost of tickets of course). If these rides are Test Track, Soarin, Expedition Everest or the Safari, etc then yes it is necessary. But if you really don't care if you ride them or not...you have the option of waiting or not riding. While September is not as busy as some. If it is a Friday night in EPCOT during F&W...yep nice to have a fast pass.
My parents really loved being able to walk right on. They are healthy but in their 70s so we would get the three in quickly, head back to resort or meander around to eat, , then out again at dusk for the parades and fireworks. They loved it. FP+ lets you be spontaneous, you only get three, you get to wait on the others. I just prefer to have my three favs for each park reserved.