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.
Which is exactly why DirectX was added to Blitzmax in the first place -- the original launch was OpenGL only, and it took a lot of complaints about speed issues on windows to convince BRL to add DirectX as well... But it was fairly clear that that was never the original intent.