OK, I'll take a stab at it and suggest that like so many other things, spontaneity is in the eye of the beholder ... and perhaps the wall here is between quantity and quality ... that's not exactly it, but hopefully you'll see what I mean.
Group 1 spends 6 hours in the park with 3 FP planned. They spend a total of 1 hr in transit to / in line for / waiting for FP. In the remaining 5 hours (non-consecutive time), Group 1 makes roughly two spontaneous decisions about what to ride / where to go in each hour. Some of these may have factors associated with general location "I'll spontaneously choose a ride in Tomorrowland because I want to be in the area for an overall park logistics reason that may include FP+", but I think it's still fair to call these decisions spontaneous because those kinds of factors crop up all the time, including with FP- planning.
Group 2 spends 6 hours in the park with no FP planned (we'll say they're there in 1997). To ride the same rides as Group 1, they spend a total of 3 hours in line. Their per hour rate of spontaneous decisions is otherwise similar, except that they only have a total of 6 (as opposed to 10) spontaneous decisions made.
Group 3 spends 6 hours in the park in the FP- era. Because they are riding major headliners, we will assume that all their return times are 1-2 hours in the future (for simplicity, and not an unrealistic assumption). They spend the same 1 hr transit to / in line for / obtaining their FP-. They have the same 10 other spontaneous experiences as Group 1.
I think most people would argue that Group 2 has a completely spontaneous experience, but could also see how the people in Group 2 might have thought that Group 1's experience is more spontaneous as it allows more opportunities for spontaneous decisions even though part of the day is planned.
I think most people would also agree that Group 3 has spontaneously made all their decisions whereas Group 1 has not. For some people this means that Group 3 has unequivocally had a more spontaneous experience, as an external observer. However, I think there are a significant body of people who, as the participant, would say that the Group 1 experience feels more spontaneous to them than the Group 3 experience. These are the people (and I can be one of them so this is not a statement made to be negative) who can't just "Let It Go". While they are waiting out their first FP- window and doing their first set of spontaneous things, they are thinking about and weighing the various options for what FP- to get after they use the first one. This is not a choice. This is the way their brains work. While they are waiting out their second FP- window, they are, at least part of the time and at least partly consciously, planning out what to do after the second FP- is used. For these people, the fact of having the FP+ rides already set allows them to feel the spontaneity that applies to the rest of the decisions, instead of feeling that even when choosing things on the spur of the moment part of their brain is busy making plans, backup plans, and backup plans to the backup plans.