So what was the point on releasing Calypso?
She didn't actually make any difference at all.
shareWell, they intended to use her to wipe out Beckett and Jones using her powers.
But Calypso wanted to be free to destroy the Pirates who bound her and then she would be with her love, Davey Jones. but after Will tells her that Jones was the one who helped bind her into a mortal form, she decided she wouldn't help anyone and unleashed a storm to curse them both during the battle.
.. she failed in destroying either of the two ships that the storm affected
for some reason the armadas of both sides were able to sidestep the storm just leaving pearl and dutchman to fight it out in the storm .
... in one really drawn out and increasingly silly sequence!