Actually, I think the user shouldn't care about what's used internally, as long as things remain simple and fast. There's really no point providing several back-ends for either audio or rendering, I just have to make SFML work for everyone.
Choosing the rendering API would make sense in a next-gen 3D engine, where one API can be slightly faster in some cases, implement some specific features, or have bad support on a platform; but in the case of SFML it would be useless in my opinion, as OpenGL is more than enough for the 2D stuff SFML does.
Regarding audio, I can't see any reason to choose one API or another as long as it works fine.
Developing flexible / modular / super powerful stuff is great, but sometimes there's just no point doing it