Under the idea of initial phase of FFXIV, you need level both to create your ideal WHM or BLM. CON has element-specific actions including both enfeeble and damage types. THM has less but more general light/dark enfeeble and damage actions, plus status-specific enfeeble and DOT. To make a great magical damage as a BLM, you need combine actions from both classes to enfeeble a target's elemental resistance and enhance your own magic potency. Also, to heal your party members without delay, you need both fast cure and regen effect from sacrifice.
Now jobs will be introduced in the future. According to the released info, you may still need to level multiple classes to obtain one specific job title.
