Hey Word,
We have hashed this out several times, but I will sum up the general consensus for you.
Now we don't KNOW how they will roll the system out, but these are the thoughts.
If they are pushing FP+ and claiming people can reserve their favourite rides 60 days in advance, they are going to need to make as much capacity available for the prebooking. Imagine how people will react if they go online at their 60 day mark, or their 50 or 45 etc etc ... and try to book, but all FP+s are gone ? I for one, as well as many others, would cancel my trip, probably after some long and hostile conversations with Disney CMs.
The pressure is to make pre-booking available.
If a day visitor walks into the park and can't book FP+s because they are gone, well, they are already in the park, they will just ride standby.
Disney's whole goal (as stated by Disney Execs) is to have people LOCK in to their
Disney vacations, to have their FP+ selections made as far out as possible so they spend more time at Disney and less at other places. They can only accomplish this if you are able to select your FP+s. In advance.
We KNOW, there isn't enough capacity for everyone to book the FP+s they want for everything they want (heck that's why they brought in tiering). If they are holding back FP+s, this leaves even less capacity for pre-booking.
They can not achieve they goal of the system, if they are holding back FPs for the same day, and in fact, will encourage people to go to other places if they are holding back for same day FP booking.
Is it possible they will continue to hold out FP for same day ? Yes, it is. As was noted I think they are currently doing this, I think the Hub mentioned something. So maybe they will continue this, but it appears to go against the entire premise of the program.