FP+ for shows and attractions that don't need them is partly about *perceived* value-- hey, look at all this great stuff you can get FP+ for! What actual value that has is debatable and, as others have said, it depends on the guests.
But let me make an argument that FP+ for shows and minor attractions does add value for everyone using the system. It allows Disney to spread out the total FP demand. By dilluting the pool with "unnecessary" attractions, more FP *return slots* are available for major attractions because a percentage of users, due to ignorance or even personal preferences, are choosing the minors instead of the E-tickets.
Space Mountain can only have X number of FP slots per day due to fixed capacity-- there's a point where you cannot load more than a fixed number of riders per hour. I would also argue that the "optimal" number of FPs distributed each day is less than X-- because at some point too many people in FP lines will not only make the FP line unreasonably long, it will also increase standby times beyond tolerability.
So how do you give the most guests the most value? By spreading out demand on the FP system-- in effect, taking potential FP users away from Space Mountain and distributing them across several minor attractions instead.
Think of it this way... if there are 30,000 FP participants on a given day, and the only FP available is Space Mountain, the FPs for Space Mountain would sell out too quickly and many guests would be left unsatisfied-- without any value to show for their participation in the system.
But take that same pool of 30k guests and give them a dozen FPs to choose from, and the stress is spread across the ride capacity numbers for the entire park.
This mode of operation relies heavily on a set percentage of guests making uninformed or "unique" choices. But even those choices have some perceived value (at least when they're made) and that helps allow those who want to FP Space Mountain get they value *they* want out of the system. So by spreading out demand across capacity park-wide, the system can allow more value for everyone who uses it.
In the end it is still a shell game and the only real solution is to decrease attendance or add real capacity. What value a guest sees in his choice when the standby line for a show is just as short as the FP line is indeed debatable. How long the system can hold up over time is also up for debate, as guests slowly become more educated on how best to work the system.
Sorry for the long-winded post