This might work for a meet and greet that people only want to experience once a trip or maybe a show but I don't see it working well for a ride type attraction as the standby lines will actually get longer, exponentially longer!
The standby line acts as a deterrent when it gets too long and makes people make a decision if it is worth over an hour wait to get in it, everyone has their own cut of point. At 15 minutes maybe 2% less people get in line at 30 mins 20% less, at 1 hr 50% less, at 2hr 90% less. The longer the line is the slower it grows, eventually it comes to a point where it just does not get any longer as it is not worth it to the greater majority. If this system is introduced for a ride and people do not have to wait in line for more than 30 minutes ever it will be less of a deterrent. People will get their standby pass do other things and come back. Essentially, this will make the standby line even longer you just won't see it because people are in line but scattered all over the park doing other things until their time comes up. I could see the standby line reaching 8 to 10 hours very quickly with people not being deterred and getting more than one pass why not I can do other things while I wait. Get a pass at 10 am and it says come back at 10 pm means the standby line is actually 12 hrs long. Anyone coming into the park at 4 pm may be very limited in what they can do. If they were to do this for many attractions in a park, this would eliminate park hopping as people would need to arrive at a park early use their fast passes so they can get more and start collecting standby passes and line them up for the rest of the afternoon and evening. Hopping to another park would mean all the good fast pass pluses are gone and all the standby passes are gone for the day. Nothing to do but watch some fireworks, parades, do low demand attractions, shop and eat.
I see some who would like this and others that would not. But one thing is for certain it would take a lot of planning to tour correctly and it defeats what they have done with Fast Pass Plus as now once again people will be running around gathering up standby passes.
For meet and greets I do like the idea though, just not anything else.