I guess its because he knew that she and Will were in love and someone was going take over the flying dutchman. Since Calyso and Davy Jones were having problems, he figure that Will would take over the flying dutchmen and Elizabeth will be the new Calpso, does this make since?
TD has a point!
if he trades the real 1, then the whole operation would be put to death.
PotC is basicaly about choices.
but it takes a lil bit of time to find the real reason to their choices.
thats y i like the movie.. oh nd also a drunken-sober-crazy pirate that acts hekka gay @ times with his body language! lol