"A few DirectX-only games get ported to the Mac too, that is, they have to port it to OpenGL in the process. Like they did with Halo."
The biggest reason they did that is because Bungie used to be a Mac heavy studio.
It's becoming less and less likely for DX* games to get ported to OpenGL, especially if they are associated with MS in anyway (Look at Gabe's resume.)
That said, DX has come a long way. I still prefer OpenGL in most games, but that might be because I'm still running a GeForce3 :/