Sure. You could. But if you have a FP booked for Soarin', then perforce, you do not have one for Test Track. So while you are revelling in the glory of your spontaneity at Spaceship Earth, the Standby Line at Test Track is quickly building to 70 minutes. In reality, most people, (pre-planners or spur-of-the-moment folks) are going to make a beeline for TT when they walk into Epcot first thing in the morning, and the notion of spontaneity goes out the window. Having a single FP+ for a tiered attraction doesn't make most people spontaneous. To the contrary. It almost dictates what their first move of the day will be. This is not to say that some people won't jump in the Spaceship Earth line as they walk in the park. But if they wait 5 minutes there and 70 minutes for Test Track, they will spend more time in line than the person who heads straight to TT and waits 15 minutes there, and later waits 20 minutes for Spaceship Earth.
But there is a bigger picture to spontaneity that gets lost here. Most of the discussion centers on the "micro". But there is a "macro" to spontaneity. Do a quick search for threads asking for advice as to which park is the best one to visit when it rains. In reading those threads, you will note two constant themes. First, locals and people with experience will tell you that advance forecasts cannot be trusted. You have to wait until you are there to figure out what the weather will be. In other words, advanced planning won't help you here. The second thing you will see over and over again is that people have their own ideas as to which park is best. The whole reason people ask the question in the first place is to get good ideas where to go if their day is going to be a rainy one. But does any of that matter if you locked in your FPs for the day? Personally, I prefer Epcot as my rainy day park. By a wide margin. But if I arrive only to find that my one and only day of persistent rain is the day that I booked the MK (where BTMRR won't be much fun in a driving rain, nor would 7DMT, nor would Astro Orbiter, nor would watching the parade, etc., etc.), what do I do? Do I spontaneously decide at 7:00 a.m. to head to Epcot where the FPs for the headliners are all distributed? Before FP+, this was an easy "on the fly" change to make. "Honey! Ginger Zee says it's going to rain all day in Orlando. We should go to Epcot today instead of the Magic Kingdom." Boom. Done. It is a lot more cumbersome now, and there is really no argument to the contrary.