But the Priest Hero combination as far as I know has no difference from a regular priest, so it's not a very attractive combination to take.
It actually takes away one of the defining characteristics of a hero (the ability to die in battle) since priests can't participate in battles.
Priest/Hero is a badass class combination. It gives the priest all of the heroes' perks and none of its drawbacks (ability to die in combat). Well, it does leave you with one drawback, the inability to change subclasses again...