Sure, maybe the overwhelming majority of Sander's voters will vote for Clinton, but the 1-2million who don't may be a big deal in a close election, or may be big enough to elevate Green Party Stein or Libertarian Party Johnson into the debates, and that would be a game changer in a race where you have the two most historically unpopular major party nominees in history that may struggle to reach the 50% plus 1 needed to win.  Stein raised $1mil in small donations since Bernie endorsed Hillary. - less than a month.  She's breaking into the national media too. Just needs to get to 15% in 5 polls and she's in the debates.  Libertarian Gary Johnson is doing well too.

See:http://fivethirtyeight.com/features/why-clinton-might-have-a-tough-time-flipping-the-sanders-holdouts/

http://www.civilbeat.org/2016/06/third-parties-the-electoral-college-and-the-2016-presidential-race/