Well, I see it as a good thing. I think common courtesy is something that needs to make a BIG comeback.
If my stated return time for any FP ride states that I have between 1pm and 2pm I will show up between 1pm and 2pm. I will not get in a line at 12:30 to ride another ride because the stand by wait time is 30 minutes. As everyone knows a 30 minute wait time can, as Buzz stated, change at the drop of a hat and become 45-60 minutes. Wait times also change based on the number of wheelchair or otherwise disabled riders are loading or unloading.
I find it VERY annoying when I see someone use a FP from earlier in the day OR from a previous day, and yes I have seen it happen. As I stated earlier, I would never show up a day late for an event and expect to be allowed entrance because I wanted to do something else first.
While I understand the "If they didn't show up at their stated time those in said standby and FP lines ride faster" thus creating a positive effect at that given moment. BUT when said riders show up late it has and adverse affect on those in line at that given moment; standby AND FP. Those pointing out that it is not simply black and white are wrong, it is exactly that simple.
I as one person do not have the right to inconvenience even one other person due to my overinflated sense of self and entitlement!!
I don't see using FPs outside the designated window as rude. And although this has been discussed a gazillion times, I will offer my explanation.
Suppose my family and yours are both headed to Space Mountain. My family arrives just before yours, but instead of getting in line, we move aside and let you go ahead of us. Would you mind? Is this inconveniencing you? Does is reflect an "overinflated sense of self and entitlement"?
Then, suppose, we keep doing the same thing. We let family after family get in line ahead of us. Is this hurting anyone? Does it even seem like kind of a nice thing to do?
As time passes, other families join ours they stand outside the entrance to Space Mountain, allowing those have arrived after them to ride before them. This continues for several hours until there is a half-dozen or so families waiting outside the queue.
Then, those families, totaling a few dozen or so people in this analogy, decide to finally enter the line. I have no doubt that for those arriving at Space Mountain at that moment, this will be perceived as an inconvenience. To them, their wait time just got longer by whatever time it takes a few dozen people to ride. But while the actions of the half-dozen families who entered the queue at that time might seem annoying, the reality is that those folks have actually been letting others ride early for several hours.
It's the same deal with using FPs outside the designated window. While those arriving at an attraction at the same time or right after the FP-user arrives, the wait time will be longer. But for everyone who rode before that moment, the wait time was shorter.
Using FPs "on time" will actually make wait times longer throughout the day.
Whether you want to use your FPs within the designated window or not is obviously up to you. But I think impugning the moral character of those who have made a different choice than you is a bit much.