Camby blocks a lot of shots. He is not a good one-on-one defender, even in the post. He is fairly average as a help defender, which is pretty lame for his size. He is only a "good" defender when compared to guys like Kaman, Okur, Chandler, etc.
It's just to difficult to project what would happen if AK were on the Spurs in place of Bowen. The defense would be inferior, but Bowen's "offense" consists of wide open set shots. IIRC, Bowen makes something like $4-$5 million a year, so he's definitely a bargain in the grand scheme of things.
It's very difficult for me to forget about Kirilenko's ridiculous contract and discuss him in the abstract.