I have very much enjoyed your discussion with Mesaboy on this subject - I think you've both made great points and it's been an interesting read.
Forgive me if you've already pointed this out - one example of an increase in spontaneity w/ FP+ is the ability to change them on the app. On both of our FP+ trips I have been able to alter FP times and experiences easily. Since these types of changes were not available w/ FP-, I would say that at least
in regards to this specific feature, FP+ allows for more spontaneous decisions than holding an unchangeable paper ticket.
I don't know if I felt more spontaneous overall, since I made a plan and pretty much stuck to it as I had with FP- trips. But I'm not going to WDW looking to fly by the seat of my pants, that ship sailed when I started traveling with little ones.
I do know that we keep having wonderful trips, and I'm already planning the next one!