I can't believe how long this debate has been going on...
OK, here is the deal as I see it, and you can take it or leave it.
A FastPass system without enforcement is just another Standby line...you get a ticket, but show up whenever you want...just like the Standby line.
By enforcing the return times, Disney is actually controlling the number of people in the FastPass queue at any given time. Without enforcement, you theoretically could have excessive numbers of FastPass holders decide to enter a ride at the same time, negating the whole purpose of having a FastPass (as it sounds like used to happen under the 'old' system, where lots of FP holders would show up late in the day).
While my family and I never experienced any waits longer than 15 minutes during our trip a couple of weeks ago, the complaints about larger queues with the 'new' rules probably have some merit. However, you have to remember the FP system is most efficient when the park is most crowded...so when the park is not so crowded, the difference in wait times will probably not be as great.
Also, I get the feeling that the shorter lines under the 'old' system may also have been influenced by those who did not know they could return outside the 'official' return window...I'm guessing that the large majority of people who missed their return window just threw the pass away, and those relative few who knew that return times weren't enforced took advantage of this situation.
Ultimately, the FP enforcement may mean slightly longer waits for those people who knew how to play the game under the 'old' rules,
but will benefit more people (the majority of casual Disney visitors) overall in the long run.