I've been on these boards long enough to know there are a lot of people with an almost militant allegiance to the old maxim: "A rule is a rule". Not only will they never knowingly break a rule (or maybe it's more accurate to say they'll never
admit to it

), they go into fits of hysteria at the thought that someone else is getting away with something.
Of course, I am speaking from the perspective of someone who is significantly rule-challenged. Back in law school, I was trained to take the black letter of the law and create as many shades of gray as possible. Even now, when I am asked a simple yes or no question I break out in hives and say "it depends". So I confess that I can easily rationalize trying to use an expired fastpass if I believe I have a reasonable excuse for not showing up on time.
Even I would probably feel guilty about collecting a bunch of fastpasses and just using them whenever I felt like it for the rest of the day. But in certain circumstances, I could justify showing up late if say, we were stranded on another ride which broke down, or my two-year old had a particularly messy BM and needed a diaper change, etc., or maybe lunch took a bit longer than I expected. I suspect Disney is not particularly stringent about enforcing this policy because it understands that sometimes visiting a theme park involves unexpected delays and detours, even for planning freaks. I also suspect that Disney figures that most people are going to try their best to return during the designated Fastpass window, and that those who show up late are probably not doing so to "try to get away with something".
So they have a choice: they can just tell everyone who shows up with an expired pass to hoof it, or they can let them in the FP line on good faith, knowing that there's necessarily a small percentage of people who probably don't deserve the privilege. When you look at the situation from that perspective, I don't think there's really even a question. Disney's bottom line isn't being affected in any measurable way because some of us are standing in a longer FP line, and I don't know too many people who will decide to not go to Disney because they don't enforce the FP rule strictly . . . so I doubt they're going to antagonize a sustantial number of guests who believe they have legitimate reasons for not arriving during their original FP window.