Collins would stand far removed ideologically from the rest of the D caucus, far more so than she is currently separated from the rest of the GOP caucus.
Collins is not far removed from the likes of Manchin, Warner, Donnelly or Heitkamp. By many measurements, she is to the left of some or all of the previously mentioned. If Bayh gets in, there would be a much healthier population of centrists on the Dem side than the GOP side which would basically just be her and Murkowski.
The problem with Collins becoming a Democrat isn't so much that she would be isolated (she already is), but that she would be much more vulnerable in a D primary in Maine than a R primary. Plus, she likes being Obama's pet Republican.