so by always using opengl, you win.
No, you don't. A considerably smaller portion of people have usable OpenGL drivers on Windows than have usable D3D drivers. So if you're making casual games where you're appealing to people who don't play hardcore games, you lose if you always use OpenGL.
In this specific instance, because Eric *wants* to use OpenGL ( and I'm not going to question his reasons, that's his choice ) then he should just use OpenGL on all platforms. But it's not a general solution for everyone. It's a balance which has to be weighed up by each individual when they come to make this choice.