I have do have firsthand information concerning "late" FP use. My cousin, Scott Bowden, works in Anaheim as a Senior Vice President in the Department of Planning, Revenue Management and Strategic Pricing - he and I have had a few conversations about this issue. It has now become a joke because whenever I call him, the first thing he says is "Fastpasses do not expire, what's up?"
Additionally, a father in our first China adoption travel group (Mike Lewinski) works for the outside firm in Buffalo that actually helped Disney design and implement the FP (they make some of the components in the machines and in the software). He is very familiar with the logistics of the FP and how they work to provide better guest movement in the parks.
To answer the question about why they print an end time and not simply the earliest time you may enter the FP line:
Because someone realized that people need guidelines. Seriously, it is because having a start and finish gives an appearance of a "reservation" instead of just giving you a time to return (which just seems like you're still waiting, just not in the line). People like an appointment. I am not kidding – it is no more scientific than that.