In ancient Greek religion, clergy do not act as representatives of the Gods, or as people who speak for the Gods. In other words, we let the Gods speak for themselves. We don't presume to know what a God thinks, teaches or desires, because that would be hubris at best.
However, that's not to say that the Gods can't send messages through people, namely their clergy. After all, the Oracle of Delphi was a Priestess who spoke the words sent to her by Apollon. She did not speak for Him, but she did relay His already spoken words to other mortals.
The Gods are above our plane of existence, and thus they use a variety of channels to communicate with us, although of course, nothing stops them from coming and going to and from any realm they so please, and at their own wills. But I also think that the Gods have a vast array of duties and functions, and manifest in different ways, to different people, and for different reasons.
But absolutely, they can speak through their Priests and Priestesses. For all we know, they could be speaking through me now as I write this, so that humanity can know more about them, or about a specific topic.
Clergy spend an exceptional amount of time being devoted to the Gods, and I think most of us in the profession today would choose it as our only job if it paid enough. And people who spend the most time with the Gods, will naturally develop a channel to them, but such a channel can also be reversed.
I think that's one reason we need clergy in Hellenism. Not to tell people what to believe or how to live, but to be a source of comfort and knowledge. I know that, as a Head Priest, there is no other clergy that outranks me, so I literally have no one to talk to for counsel. I go to my wife who's an Oracle, or the local Seer in the next City. Hellenists today don't have a lot of support to turn to.
Yes, it can absolutely happen, and perhaps more often than not, so the words of a Priest or Priestess should not be automatically discounted. You should definitely consider that you're talking to a unique person, and perhaps even someone that the Gods have put in a clergy role for a reason. In the very least, a studied member of the clergy knows what they're talking about when it comes to Hellenism.
In the Goodness of the Gods,
I'll see you at the next Herm down the road,
Chris Aldridge.