For whatever reason we as fans love dreaming about 2 HBs on the field at the same time, like when we had Lynch and Jackson or Jackson and Spiller, and these conversations take place and then nothing comes of it. I can't think of an offense anywhere in the league that has used it to any great effect in the last 15-20 years.
On 3rd & 1 Josh, Diggs and the one HB give you three different running options. Instead of a second HB I'd rather someone who makes us more dangerous on the ground with their blocking (Lee Smith, Kroft, Knox, a 6th OL) or who spreads the defense out (ie Beasley). A second halfback to some extent is wasting a man, seems to me.