Zoro voices his opinion, just like everyone else on the crew. Luffy doesn't value him any more or less than the rest of the crew. He'll go to the depths of hell for anyone of them. That's what it means to be a crew to him. The Strawhats are special because their bond with their captain is as unique as their captain himself. They all follow Luffy because believing in his dream gave them permission to believe in their own. And everyone on that crew has a role play.
Zoro is not an authority an figure, he's a trusted figure. He's Mr Reliable, and not even Sanji could deny what a kind, loyal, protective and genuinely selfless friend Zoro is. They trust him because they know he only has the best interest of the crew at heart. Also, Zoro is not one to talk frivolously or challenge his captain for the sake of it. If he's voicing reservations or opposition to an idea, that's going to have weight to it.