You know, I'm a stats guy by nature, and I'm trying to do the math on this, and see how much it actually affects people who are not in the park early. It seems to me that if they have crunched the numbers properly - i.e. how many FP+s have been booked, the number of people held in the 30-minute standby line, and the throughput of the M&G, being given a return time shouldn't really alter much, except that you're free to go do other things.
What I mean is, again, if the numbers are correct, if you get to the line at, say, 11AM, and are given a return of, say, 3PM, that indicates that if you had to stay in the standby line you would have had a 4 hour wait, and wouldn't have gotten through till 3PM anyway. There is a limit to how many patrons can get in in one day. They have to cut off the line at some point - right? Assuming a constant flow (which is, admittedly, a big assumption) if you showed up at 5PM (a real late sleeper, like me 10 years ago) you might be SOL even with an unlimited standby line.
I'm not factoring in the number of people that bail from a never-ending line, as that is pretty unpredictable, but other than that, what am I missing? The dip in patron flow that occurs later in the evening that people have alluded to earlier in this thread as something they could count on vis-a-vis getting in standby might be the reason they are (could be) re-opening that line at 8PM.
I know it's a work in progress, and seemingly changing things up on the fly will always unsettle people (me included) but this just might be a real positive move. Just my $.02.