Interesting concept ..
I definitely feel the current culture of "ensuring" getting the best spot is getting out of hand.
This happens everywhere .. whether it is going to a movie, going to a Black Friday sale or seeing your local 4th of July Fireworks.
The amount of time people consider "worth it" to wait for an event or attraction is incredible to me.
But the problem is a downward spiral.
Since people show up 30 minutes beforehand .. then other people show up 45 minutes before to be first .. then other people show up 60 minutes before next time to be first .. etc. etc. Until years later (as people get "wise") you get people who are willing to camp out 3 hours before a show ...
Disney wants to alleviate that traffic jam, so I don't see a problem coming to the park and ensuring you get a seat and knowing you don't have get there hours ahead of time to watch a show. Sure .. you can't pick it .. and I would argue that a majority of the people watching the show it is their first time and they won't care where they sit.
This is better than trying to use a FP+ for a night show that you may not go to (due to being tired, weather, hopping to another park, etc.)
IMO, they need to find a way to "disconnect" the evening shows from MM+...make it so that you can book a FP+ for them (maybe not as far in advance as everything else?), but that it doesn't count against your FP+ allotment for the morning/afternoon.
THIS ..
Grabbing a FP+ for a night show (with your initial 3) severely puts you at a FP disadvantage to someone else who schedules all three of theirs before lunch and goes to get a 4th, 5th or 6th Fast Pass for the afternoon/evening (and may still even be able to a FP to a night show!)