Interchangeable backs on medium format SLRs - when/what/why do you use them?

Nice summary.

It is a "when/what/why" question - not so much a "when not/why not" question.

An advantage of interchangeable backs for me is that, for the venerable Rolleiflex SL66, the potentially troublesome original back which uses the film thickness to trigger frame alignment can be replaced with the last version for the SL66SE which uses the simple and reliable match-the-arrow system of loading.

Interesting. My older brother sold Rollei gear and had a couple of SL66 kits himself, and complained about the troublesome backs.

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.

Black & white, color and infrared film. Different speed films.