While people have stated we can fill any roll, they haven't mentioned the synergy a priest provides. In the hands of a competent player a priest's versatile tools do more than let you do any roll. Those tools helps other classes do their rolls. For example, a holy priest grouped with a mt healer can help the mt healer via PW:S to buy the MT healer time to land a big healer or to help get through a healing debuff effect. The priest can use GS to help the MT healer get through a period of high damage. A holy priest can place a lightwell next to the other healer, so the other healer does not have to worry about spending time healing themselves. All of these examples can be done with a 1.5 cast/gcd. This means a priest can still do the raid healing they are suppose to while helping the MT healer keep the tank up through rough spots. It doesn't take too much imagination to see other ways a priest can use their tools to help any role while still accomplishing their own.