The fact is, that by making judicious use of FP (i.e. using it outside of the window), you were able to maximize your day and see and do more with less wait. If you chose to always use your FP within the window (either because of ignorance of the leniency or because of bullheadedness), you would experience less attractions throughout the course of the day and wait longer in line.
Of course those who did not use FP to the greatest efficiency would notice no difference in their day, because they were not maximizing their days to begin with. For those that did use FP wisely, this new policy will be a reduction in the amount accomplished during a typical day, and a lessening of vacation value.
Len Testa, of touringplans.com and a reliable source when it comes to efficient touring and estimating wait times left this comment on the USA today article about FP enforcement:
"I don't see how enforcing FASTPASS return times will result in shorter lines, as Ian says. For example, morning guests can no longer "bank" the savings they get from obtaining FASTPASSes early and riding late; they have to return and ride during the morning. As a result, there's less time for visiting other attractions in the morning, when lines are low. Every simulation I've run says guests will experience an increase of at least 4 minutes over the course of a day, and the average is somewhere between 15 and 30."
Here is a link to the article
his comment is the fourth one down, at the end of the article.
I'm not pleased with an average increase in wait times between 15-30 minutes, waiting longer and accomplishing less will certainly negatively impact my vacation, and I can't imagine why anyone would consider it to be an improvement.