Well, If you think that 8% is not worth programming for, you're software will not that successfull as if it used OpenGL. It seems to me that everyone just thinks DirectX is the only possible solution, and it handels input, sound etc. Well, this can also be accomplished with various of libraries like Allegro, ClanLib, SDL.
Besides, those 8% not using MS windows today, may be 20% in a few years. You can't rely on todays situation. I can agree that it's convenient to have a DirectX syntax to everything in your game, but I believe that you have to search an API that you like for yourself. Be open minded, and try out many, look into the documentation of the API's.