A mingw compiled program under Windows also needs the correct dll for the mingw runtime.Not quite.
"GCC does not include a C runtime library. This is supplied by the platform. The MinGW port of GCC uses Microsoft's original (old) Visual C runtime, MSVCRT, which was targeted by Microsoft Visual Studio 6 (released in 1998)."
MSVCRT (not to be confused with, say, msvcr100.dll -- a runtime specific to Visual Studio 10.0/2010) is an OS component that ships with Windows itself (the end-users don't have to download anything extra). The users also don't have to update to anything extra (since 1998). In other words, all of Windows users since 1998 automatically have "the correct DLL" to run a C/C++ program compiled with MinGW.
That's not to say there are no downsides in this particular case: http://mingw.org/wiki/C99