Mainly because I set up key arrangements differently based upon stance and specific fights. So sunder in one stance bar is not in the same place for a second stance as I may not use it as much. Sunder may be a bad example (HS or Thunderclap are probably better) but you hopefully get the point. I organize buttons based upon frequency of use on a per stance basis.
The new macro mechanism is nice but I've not been particularly pleased with the visuals of doing it this way since the cooldowns and icons are used only for the first spell in the macro. There are some interesting macro ideas but then the same issue applies; personally I'd want that ID used only once regardless of where or how many places I put the macro in my UI.
The user already got one bar with 10 buttons, which means he's using IDs 1-10. The user then presses "Create new bar", and adjusts the number of buttons for Bar2 to 5 using the dewdrop menu. He's now using IDs 1-15. He now sets Bar2 to change page on stance. He's playing a character with 3 stances, so the system will now assign 3 IDs per button on Bar2, which means he's now using button IDs 1-25. He now sets Bar1 to change page on modifier key shift down. He's now using IDs 1-35. If he decides to delete Bar1, he will now free up IDs 1-10 and 26-35.
This is the approach pubcrawl and discord action bars uses.
I feel it is better to share Button ID's when it is the same action instead of assigning them uniquely and replicating the spell. If I understood your description correctly, my warrior wanting to use Sunder Armor in a stance bar and in each of 3 stances would consume 3 ID's for the same skill. Instead, ID's should be mapped into the 120 ID's Blizzard provides and then Button Frames simply share the ID needed. This is the approach FlexBar (and now FB2) took. It makes for a challenging configuration scheme but is something I've wanted in various Bar mods. Something like a Spell Book metaphor but with the book representing the 120 available slots would be ideal.
The other challenge I've faced is having instances of playing on different computers. As a result of this, I set up my buttons with the default UI and then use FlexBar on my home machine to manipulate buttons into a layout and logic I desire.