hopefully the key to your scenario is that they will allow same day FP+ in addition to the initial 3 prebooked FP+'s. otherwise, yes, those who do marathon days will be shorthanded and be stuck with standby lines. of course, maybe stand by lines will actually improve...everything is yet to be seen.
i could see where maybe the whole 3 FP+ prebook thing is being interpreted as going to be 3 FP+ only when maybe it will be a max of 3 FP+ prebooked but then allowedo add FP+ as a FP+ is used up, just not allowed to exceed more than 3 FP+ bookings at a time. if that is the case, it could be great, cause as it is now, we are only allowd 1 FP at a time and no stacking of FP's for late returns. it alsow would make it so that if you plan to be in the park all day, it's best to get your FP+'s for earlier slots so that you can keep adding more as the day goes on. in this scenario, i would think going RD and doing stand by would be optimal wiht using the 3 FP+'s up during late morning mid day so that more can be added in afternoon/evening slots. again, that is IF they allow more FP+'s to be had.
I agree that this is a KEY piece of information, but we really don't have any reason to think that they will continue to allow for more then 3 FPs per day. While we do have reason to think that they will not. We have heard from several people that part of the reasoning behind FP+ is to limit the number of FPs being used in order to shorten stand-by lines. While we have had many people say you will be able to pre-book 3FPs for your favourite attractions, and alter them as you need, but I don't think we have seen anyone add in that you will be able to pull additional FPs through the day... which, you would think that they would have ...