The big thing that I'm seeing with this is the possibility of the FP line getting overloaded with people. Yes, those with FPs are supposed to get through quicker (and before those in the standby line), however we know that that doesn't always happen. They also don't let all FPs through all the time; they let a certain number through, then make the FP line stop while they let some of the standby line through.
Why do I think this? Each FP ride gives "X" number of FPs for a certain return time (for argument's sake, let's say you are there at rope drop and get the first return time of 9:45 and that there are 120 available FPs per return time). Once the 120 FPs are distributed, the increment changes by 5 minutes (I think a couple do it in 3-minute increments). So, now you have your FP and you return at 9:45 to get on the ride. As the rule is now, the 120 people that have a window of 9:45 - 10:45 can return ANY TIME between 9:45 and park closing, 120 more people can return ANY TIME between 9:50 and closing, etc. Right now, you'd probably get right on if you walked up with your 9:45 FP return at 9:45. Now, if the rigid enforcement takes place, there are 120 people that MUST return between 9:45 - 10:45, 120 people that MUST return between 9:50 - 10:50, etc. This could create a bottleneck in the FP line with everyone trying to return at the same time because they are forced to. Not to mention that it goes against Disney's original intent to allow people the freedom to spend $$$ at other places.
Maybe if they added more FPs to a return window, but then also made the gaps bigger. In other words, maybe give 200-250 FPs at 9:45, but then make the next return window 15 minutes later.