Dick, you are correct that software exists which appears to convert midi to MP3 in a single step, but I believe that what it is actually doing is playing the midi on an internal synthesizer and recording the output, but doing it all internally so it seems to be a single step. Its not actually converting the midi file. And as you indicated, mostly the synthesizer is very poor quality, so the end result is not great. A bit like the result you get when you play a midi on your computer.
Mike