With the functionality provided by the main SDL library, it is possible to rewrite your own mixer (SDL_Mixer), font parser (SDL_Ttf) and image processor (SDL_Image) if you were so inclined without writing any platform specific code. However, to use the capabilities of your network card, you would have to introduce platform specific code or use SDL_Net or some other networking library.
As a result, I believe it would make SDL a more complete platform layer if you were given networking capabilities out of the box. Furthermore, it would be nice to not have to write any platform specific code or introduce other libraries (even if the networking code is relatively trivial). Ultimately, I was wondering if it would be possible to merge the SDL_Net library into the main SDL code base.