Ok, Doc, I *think* I understand your analogy, but just in case I don't...
I have a MB, I have a package that has 8 day tickets, and I have 2 KTTW cards from last year's Sept/Oct trip-all tickets are used up on those. How many "sets" of FPs do I have?
I don't see us actually using much past the 3 FP+ selections we have , but I'm curious.
The extra KTTW cards have nothing to do with the analogy. This was about having multiple media being able to let you in to the park.
The following assumes nothing changes from how they have handled things so far.
You have your bands. They will not get regular Fastpass, but can use your FP+, open the room door, charge to room, and let you in to the park.
You will get a KTTW card. That can do everything the band can do (right now - future functions for the bands yet to be seen), and can be used interchangeably for that. In addition, it can be used to get regular Fastpasses in the normal fashion.
Your old KTTW cards MAY be able to get Fastpasses as well. I'm not sure what the criteria seems to be that allows them to work. Not all work. It may be that they are RFID, or they were for certain resorts, after a certain date, or something. If they do, they get Fastpasses independently from your current KTTW card and each other.
The old Fastpass system doesn't work like the "multiple key" system. It only knew if a "specific" key was used to get into the park. It hasn't yet been programmed to understand the multiple key system, and probably won't. When you used it, it would do two things - ask the admissions computer if it was used to enter the park, and then look at the Fastpass computer to see if you are eligible to get a Fastpass at that time (based on when you last got one).
In order to allow the KTTW cards to still work if a guest used a MagicBand to enter the park, they took a shortcut - just allow KTTW cards, whether they were used or not. For guests currently staying at WDW resorts, it wouldn't matter - most would have used the KTTW card anyways to get in. But AP holders wouldn't, and they'd normally use their APs to get Fastpasses. But if they had a KTTW card as well, they could get more.
And, as it turns out, KTTW cards from past trips - as it seems it doesn't have any way to know if the card is current.
All solvable with programming and access to the appropriate DB records - if they wanted to.