I've got four 120 backs for my RB67s, depending on circumstances I'll take three to cover B&W, colour neg and slide, occasionally have the fourth loaded with B&W IR if I'm so inclined.
Advantages are obvious... you can shoot to each film's strength on different subjects at different times of the day without wasting film. During bright midday sun you can roll with B&W and save the Portra and Velvia for more worthy subjects in the morning and evening light.
Lack of interchangeable backs on the M645s is easily my least favourite aspect of those otherwise excellent cameras.