OpenGL drivers on Windows are still pretty far behind DirectX drivers, especially on the kind of low-spec hardware (read: Intel GPUs) average customers have. If you use OpenGL instead of Direct3D you might be paying as much as a 50% performance penalty (we certainly see about that when comparing our GL and D3D renderers for the same scenes)
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OpenGL drivers on Windows are still pretty far behind DirectX drivers, especially on the kind of low-spec hardware (read: Intel GPUs) average customers have. If you use OpenGL instead of Direct3D you might be paying as much as a 50% performance penalty (we certainly see about that when comparing our GL and D3D renderers for the same scenes)