Problems with Miller:
One, she has little statewide stature. Although she was once SOS, Miller isn't well-known by most Michigan voters and is unlikely to sway many votes.
I'm sorry, but that is completely false. She has pretty wide name recognition and is liked by a large portion of the state's electorate. A poll back from 2006 showed Granholm beating DeVos 49-33 and only beating Miller by 4%.
As for the question of her swinging voters, It'll make MI closer but I doubt it'll flip the state to McCain.