I'm reasonably certain that this has started to change in MSVC 2013. They have stated that they have improved the C99 library support and unless I'm mistaken they have finally permitted mixed code and declarations.
They have implemented the C99 parts required by C++14 standard, plus few extra parts required by some well known open source projects. It remains open if they will ever implement the rest in MSVC++.
However, VS 2015 will have clang integration.