It's like you have learned to write, but you can write only in english, although you speak many languages.
No, it's nothing like that. A better analogy would be that you speak many languages but you only use English when you're in England. Which is a pretty good idea when consider that most English don't speak anything else. Similarly, using DirectX on Windows is a good idea because Windows drivers don't speak OpenGL very well. It doesn't - as your analogy incorrectly suggests - have any limitation as regards working on other platforms. That's what the platform-specific compiler switches are for.
Of course, if you're using some bleeding-edge 3D engine which doesn't work anything other than Nvidia and ATI, and even then only if the drivers are right, then I guess it won't matter.
Horses for courses.