My thought is that he's a good boogeyman for the Ds, in much the same way HRC was for the Rs. Just say the name and you'll get a strong negative reaction.
So that drives turnout. While it is still important who the Ds nominate, Trump is going to be the main motivator for that constituency. I also believe there are a lot of Rs and independents that would vote for a Romney or Haley that might decide to stay home, or even vote D despite their voting tendency/history.