It doesn't add much to the range over the three speed. I don't have the calculation handy, but given the way it works, it can't add that much (the difference between a 13 and 15-tooth cog from memory). The "half-step" shifting is OK but still a little clunky, IMO.

I also recall that you can do the six-speed as a retro-fit if the regular three speed fails to provide enough range. So my inclination would be to keep it simple -- especially if one doesn't have a particular purpose for it -- and modify it as needed later.