I was definitely one of those people that returned late on Fastpasses. It was far more convenient to tour as efficiently as possible for as long as possible, all the while building up a stockpile of Fastpasses throughout the day.
Having said that, I doubt that late returns were that much of a drain on the system, nor do I think it would be too problematic to continue the old policy after the introduction of xPass. When you add in the additional Fastpass attractions coming on line (Little Mermaid, Dumbo, Seven Dwarfs Mine Train, The Seas with Nemo and Friends, Spaceship Earth, The Great Movie Ride), there will be more Fastpasses distributed across more attractions on any given day.
The reason I think they're doing this is because the xPass excuse is an easy explanation for the need to start enforcing the end time. It's not likely that this was necessary, but it's a reason that can be stated publicly. If it is justified, I would speculate that they will be specifying a short time period as the selling point for xPass in any associated literature.
I really think that the end time enforcement is really only necessary at certain attractions. It's certainly not necessary at the attractions where the return time is only 40 minutes out. Those attractions are distributing Fastpass at a rate slower than the programmed allocation, as such, the risk for backup is minimal. There's a way around this for it to be more accommodating, and still accomplish what Disney is looking to accomplish.
Attractions like Toy Story Mania could have return times that are 8-10 hours out - the window for this should be an hour.
Midrange demand attractions like Expedition Everest could have return times that are 2-4 hours out could have windows of 1.5-2 hours
Low demand attractions like Jungle Cruise or Buzz Lightyear regularly have return times that are 40 minutes - 2 hours out. They could have windows of 2-3 hours.
This is very easy to accomplish both from an implementation standpoint, and a guest comprehension standpoint.