Thinking time, gentlemen!
Heph believes that the Door should be opened because of how the System has responded to simply our possession of the Key. They REALLY do not want us to open the door, so it HAS to be something that is either extremely helpful to Mankind, or extremely detrimental to the System. That's justification enough for Heph.
My points:
1. It seems like the best option to survive right now. We cannot reasonably make it back if we turn away.
2.The System REALLY doesn`t want us to open it, so it should be something bad for it. We don`t know if it will be bad for us as well, but we could take a gamble based on the information we have.
3. What is the point of having the key if we not open it? Melkor is a very curious person, and if we don`t open the door he will regret it later.
4. Related to above- it is quite likely that what is inside will greatly increase our knowledge and understanding of Matrix and Machines- why else are so many Exiles interested?
Sirin is very curious as to what is behind the Door, as I am sure everyone is. Everyone she has talked to so far as kept the Door as a big mystery, and she wants to unravel that mystery and get her questions answered. She's laid her life on the line for this Door, she'd better damned well get her answers if this thing opens!
Klez...is unsure. While the System has panicked about their possession of the Key, Sennacherib and the Hunter have both encouraged the opening of the Door. They're Machines, but they're not with the System. It could be something that would benefit them, rather than us.
Also, The Oracle, Lo Qi, and now the Hunter have stressed the point of being completely and absolutely sure of our decision. However, we know so little about the Door that Klez could not reasonably say that he's sure of his decision to open it.
Therefore, Klez does not think the Door should be opened, but is not about to stop the others...
"Wait," Klez says to Hephaestus, watching the Hunter. "I don't think we should open it until we get some answers from our Hunter friend here."