Let's think about this mathematically...
Let's say for example that we are talking about a random E-ticket attraction that has FP at MK (doesn't really matter which one for this argument).
Now, its a standard summer day and the park is open from 9am-11pm. So, we have 14 hours of ride/park operation.
We need to figure out the number of fastpasses that are distributed. Ruling out most of the first hour, and most of the last hour of the day, we're left with about 12-12.5 hours of FP distribution during a day. To round things off, let's just say 12 hours of FP distribution.
As a conservative estimate, let's say that about 80 FPs are set to be
distributed per 5 minute time slot during the day. However, in practice, this is the maximum number of FPs distributed. For an attraction like TSM, all FPs could be distributed. However, especially in the offpeak hours, the FP time may rollover without distributing all 80 FPs. So again, as an average, let's say that about 70 of these 80 FPs are actually distributed per 5 minute window.
This leaves us with...10080 FPs distributed during the day for our attraction.
Most people do not take advantage of the system and use their FP at the proper time. Let's say this happens with roughly 80% of FPs. (If they don't use it within the hour, they are at least close enough. Within an hour).
That leaves 2000 FPs out there. There are also people that may never use their FPs, but we'll just say that there are 2000 people that actually use their FP late.
Now, we have a 14 hour day. But realistically, if you're using your FP late, we probably only have a 10 hour window of late FP usage. So that comes out to roughly 200 people per hour. So, therefore, every hour, (after about 1pm or so), we expect that there are 200 people using their fastpass drastically late (2+ hours).
Obviously, this is a rough average. There will peaks and valleys in this distribution throughout the day depending on park events. One of the most popular times happens to be right after the fireworks (ESPECIALLY at Space). So during the hour immediately following the fireworks, we'll say 400 people use their fastpass late.
As an estimate for the average OHRC, let's use 1600. Some are lower (TSM), and some are higher (Splash).
We can now calculate how much longer the standby line would be due to these late arrivals. Assuming all 400 people came to use their late FP AT ONCE, the standby line would be only 15 min longer.
For our average of 200 late FP users, the standby line is only about 7 min longer due to late FP users. (Assuming they came all at once)
In practice, they aren't going to come all at once, so the effect on the standby line would be even lower (unless the line is greater than an hour).
For example, based on our average, if Attraction X has a 30 minute wait, we can expect only a 3 min increase due to late FP usage.
Therefore, we have learned that the net effect of late FP usage is trivial. This is why Disney has chosen to not argue with the guests for the last several years. It simply isn't worth the argument.
If FP pre-planning were done, the number of FPs distributed would probably go up and therefore the effect would be higher. At that point, Disney may begin to enforce its policy.
