Quite debatable in deed. I've done one trip with MB. The advance planning was the opposite of spontaneous. And I wouldn't have been able to secure FPs to SDMT, TSMM and a few others without the advance planning. However, we've long had to book ADRs for popular restaurants way in advance. At least FP+ is only 60 days, instead of 180.
Staggs' comment is correct in that we did feel like we could be more spontaneous and flexible in some ways once we arrived. We didn't have to get there at park opening to get FPs at reasonable times for popular attractions. So we could sleep in if we felt like it. We didn't have to split up with one person going to get FPs before meeting up with others elsewhere -- instead we could all stay together. I could make changes to dining and FPs while standing in lines using the app, and I could choose times (within availability) for FPs instead of taking whatever the machine spit out. All of this allowed us more freedom and spontaneity.
But that was only available to us because we were willing to decide, 60 days out, which park we would go to on which days. We did change a couple of those days, and while we ended up with reasonable FPs and dining, it wasn't easy and wasn't what we hoped for. And that shows the downside and counter-argument to Staggs' comment. If you are the type that doesn't want to decide which park to go to in advance, doesn't buy tickets in advance, or doesn't plan your trips much in advance, you lose. So planners get more spontaneity during their trip, while those like to be more free-wheeling about their trip get less freedom in their free-wheeling.