The temple is a generally useless building since it provides only healing. The priests are likewise problematic; their vulnerabilities and slow speed make them hard to coordinate with many effective troop combinations. As a result, this build seems to be ignored in most high level play. While I don't wish for it to be a must build structure, but it should be at least viable in some cases. Age of Empires II made it a useful building by two ways: first, it could generate income with relics; and second, monks could be used to defend against costly units like knights through their conversion abilities.
Conversion can seem cheesy in a game, something practically game-breaking if battalions were introduced, but it makes a valid point: priests should be viable units to train outside of their healing abilities. Religion played a focal role in life of the ancient times and should not be underplayed.
One way that temples could be more useful is to introduce patron gods to specific temples. For example when a temple is built, he player could dedicate it to Athena for a defence and loyalty buff, Poseidon for a horsemen and ship buff, and Hermes for a commerce and movement speed buff in the case of Athens. There also could be upgrades to improve the abilities of priests beyond healing.
Generally though, if there are specific gods available to players unique to each faction, that would of course make balance harder, especially in the case of contrasting the paganistic traditions of most factions to that of the largely monotheistic Zoroastrianism that Persians practised during this time. Still, I think that an approach like that would be much better than the current iteration of temples and religion. That said, I would be interested to hear alternatives to this option.