I think another reason the Obamas are being trotted out is because control of the Democrat party machine hinges on who the Dems perceive as supporting the party. Of course, if Hillary wins, they will want to call in the chits or keep her silent about their own scandals; but if she loses, they want to be sure they are not blamed for the loss by her supporters and can step in and depose the Clintons of their party control.
I have another view on why the Obamas have been trotted out. Some people believe Moochelle may be drafted to run if Hillary continues to decline in the polls. Perhaps this is a trial run to see how the crowds react.