Thank you, thank you, thank you!!!
I hadn't thought about this. For instance, under the old system if you arrived at rope-drop at Epcot and hoofed it over to Soarin' for fastpasses, even if you were in line at the machines at 9:30am, the return time would often be 11:20am. This means you couldn't get fastpasses for Test Track until 11:20. So at 11:20, we would stand in line to get fastpasses at Test Track that had a return time of usually 5:00pm. (btw, I have one child who is autistic and one with ADHD, so standing in line for 60+ minutes for anything is a farce) This has worked okay for us because we used the down time in the morning to eat the breakfast that we had to skip in order to get to the park on time and we used the lunch hours (when everyone else is eating) to tour World Showcase and then had a late lunch. Honestly, we had it down to a science for a few years. However, last year we had the bright idea to save money by cutting a day off of our vacation and swapping one of our park touring days for a day at Blizzard Beach. Our plan went downhill fast at Epcot. We saved Epcot for our last day, and at 5pm we went to use our fastpasses at Test Track and it was down. We couldn't stay to see if it would come back up and so we never got to ride it.
I could see, however, how having a fastpass ahead of time for one of the headliners could save you time! If you have a late morning fastpass for Soarin', for instance, before you even get to the park, you can make a bee-line for Test Track with perhaps a 20 minute wait (which is possible for us) and still have time for breakfast!
Even if there is a break-down, I've heard that you can re-schedule for another day in your trip.