I actually believe that late FP use as only minor effects on the FP line, and on average makes the standby line faster...you do have to get away from the concept that the wait time clock represents any sort of reality however, and go on the time of day you'd actually get onto the attraction based on when you entered the line.
I've gone into the technical details many times - it is all here, but it is deserving a rewrite:
Why using a late FASTPASS doesn't matter!
It doubles the wait in DL where TSM has no FP on the busiest of days TsZm gets a 35 minutes wail at the most.
The rid loads fast and doesn't need it IMO.
I think that's the first time anyone has ever claimed that TSM is a fast loader...it certainly isn't fast enough for its popularity at WDW. I wonder if the differences in wait times at DL vs. WDW have as much to do with crowd as it does to Fastpass - more on that in a second.
The situation even seemed to be exacerbated several months ago when they apparently slowed down the load rate somewhat (I heard it was due to new "exit gates", but I didn't notice any difference), and then lowered the number of FPs to keep the ratio the same.
Personally I would like for Disney to do away with Fastpasses entirely.
First of all, the parks was designed with the expectation that a certain percentage of Guests would be in lines for attractions; having FPs changed this.
True - but it has actually freed up a lot of room. Room they haven't done anything with...
Second, the FP actually is a failure from what was expected; Disney had thought that if people were not in lines for an attraction they would be spending more money on food and merchandise, but there was no uptick in sales other than price increases. It just drove people to other lines.
Also true, although I still have to believe there was SOME uptick - certainly nothing representative of what they would have liked. I definitely CANNOT spend money in a line. I MIGHT spend money if I'm not.
And third, without Fastpasses the lines will actually move faster.
I've mentioned this before...yes, the line would technically move faster, velocity-wise, and more constantly. But it would also be longer. It's essentially a wash - Fastpass does not and cannot increase or decrease the load rate or capacity of an attraction.
What it really changes is the perceptions of that wait. Waiting in a long line for 60 minutes but where you are moving frequently certainly seems a lot different than waiting in a short line for 60 minutes where it seems like you rarely move.
I think some of the nextgen additions are actually to specifically offset that "stagnancy"..."Man, this wait is sooooo booooooooring...ooo, shiny!"