Originally the elemental type had an additional effect to all your attacks, such as holy did more damage to undead, poison did a dot, cold slows ect. Apparently internal testing found that players will always pick the one that increases their damage the most, and if all the damage was equal, they would always pick the one with the best CC.
In other words, if it was between cold slowing, and fire adding a dot, everyone would want fire. If they made cold do the same damage as fire as well as slow, then obviously cold is better. Thus the dilemma.