(1) I know many people who use Linux. If you honestly don't know anybody who uses Linux, then you must not know very many people.
(2) Linux may have a relatively small user base today, but it is growing rapidly.
(3) While DirectX is used by the XBox, all the other consoles (as far as I know) are OpenGL only. When talking about portability we are not only referring to PC operating systems.
(4) Even if you are only interested in writing games for Windows, there is still no compelling reason to choose one over the other:
(4a) Direct3D and OpenGL are equally powerful as far as graphics go. If you claim they are not, then name something that can be done with Direct3D that cannot be done with OpenGL and its extensions.
(4b) It's true that D3DX provides APIs for image/model loading, vector and matrix math and some other things OpenGL does not. However, OpenGL's API may be easier to learn, especially for beginners, in part because it is not so big, it does not require you to deal with COM, it has an immediate mode, and other reasons.
(4c) You can use the DX sound, input, and networking APIs with OpenGL if you want. You might also use other libraries if they are better suited to your specific needs. You can use these other libaries with D3D, too. DX is not a packaged deal; you can pick and choose which parts of it you want.
(5) At the end of the day, it doesn't really matter which APIs or libraries you used to make your game. What matters is playability, originality, and fun. You can make just as good a game in OpenGL as you can in D3D. So it's really a matter of personal choice and style.