I think to do this would require tracking of the FP usage more closely (which would more easily be done when the xPASS tech stuff gets installed, etc.). And then you have some issues to contend with - you "reload" the FP machine with more FPs - but those FPs are for a later time than the ones that weren't used - which means MORE potential FPs used in a given period later on. That essentially creates the same "problem" that just allowing using them late would - you are just trading which people are doing it.
Plus, only a limited number get re-loaded...what should the return time be? Will there be "Fastpass Hawks" waiting for the machines to get reloaded? What happens if a group of 4 only gets 3 then because it ran out again? (oh, yeah...just claim they "lost" one...
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Far more complicated than just forgetting them, rolling in a "we know on average this many will go unused" factor into how many are issued, and/or just giving the standby line a break.