So how does this differ from doing something like;

  pandoc -o index.html < index.md ; open index.html

I was not aware of pandoc, just installed it and that certainly does the same job and a lot more.

I guess if you are looking for something solely for previewing markdown and don't use pandoc you may want a lightweight solution. Pandoc has a larger footprint and there are a few more keystrokes after all :)