@b4n, FWIW Windows filenames don't allow double-quotes in them:
http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
I guess other arguments besides filenames could though.

@ntrel Is there any way to just directly run the command string as provided by the user? I think it's kind of expected to have to put quotes around stuff with spaces, if the command string didn't have to get split up and then rebuilt, it seems like it'd Just Work, without munging any quotes or guessing at what needs quoting or such stuff. I didn't look super close at the code to see why it gets split and rebuilt so maybe I'm missing something.