What if your product is creating giant networked ovens for Nabisco assembly lines?
What if the product is an Enterprise Firewall Spam Detector IDS Bad Guy Defeater Gateway Proxy with more feature check boxes than atoms in the human body and the PM (who is a PMP, obviously) has a BA in English with a minor in interpretive dance?
What if the product is a series of experimental HIV vaccines?
Not every product in the world is a consumer webapp.
The number one skill a good product manager should have is being able to understand what the users need (not always the same as what the user wants, of course). Using the product, when possible, is a great shortcut. When it isn't possible, customer understanding is going to require phenomenal communication and imagination. That said, I would still be very wary of putting someone without some domain knowledge in charge of speccing out a super complicated product.