I'm still confused about what makes a Pirate Lord's Piece of eight special. It says they were created to bind Calypso, but the pieces are unique to the CURRENT pirate lord and not the one that was present when Calyso was bound. So it seems that a pirate lord can somehow choose his piece when he becomes a pirate lord.
In that case, it is questionable why they needed to rescue Jack in part 3 and why elisabeth had used Sao Feng's piece of eight.
- The nine pieces of eight(Jack's piece of eight, Barbossa's piece, and the rest), were the nine pieces of eight used to bind Calypso. As confirmed from Gibbs from here:
- Pintel: "Those aren't pieces of eight, they're just pieces of junk!"
- Gibbs: "Aye, the original plan was to use nine pieces of eight to bind Calypso, but when the First Court met, the Brethren were, to a one skint broke."
- Pintel: "So change the name."
- Gibbs: "What? To 'Nine Pieces of Whatever-We-Happen-to-Have-in-Our-Pockets-at-the-Time?' Oh yes, that sounds very piratey."
And no, I don't believe that the pieces of eight were chosen per say...more likely it was either stolen by or given to the current Lord.
And there's an obvious reason why they needed to rescue Jack in Elizabeth used Sao Feng's P08:
- Jack's rescue - He still had his piece of eight in his possession in Davy Jones' Locker. There is no possible way for all nine Pirate Lords to meet without all nine pieces of eight.
- Feng giving Elizabeth his piece - Feng was dying and he needed a successor(pretty much explanatory there).