I have three camera systems that take interchangeable backs , Kiev 88 , Bronica ETRSi and Bronica SQ-Ai and have several backs for them.
Depending on what I'm doing , where I'm going and what time of day I'm going , decided what film I'm carrying.
When I go away on holiday in my campervan , I'll have one back typically with FP-4 for typical daytime use .
HP-5 for evenings or areas of low light ( inside use etc ) , Rollie infrared and very occasionally, colour film .
That doesn't mean when I go out I carry them all , I might just have one on the camera , and another type in a bag or pocket .
Sometimes if I'm near the end of a roll when I'm going out , I'll have a spare back loaded ready to take over , so I'm not changing film on a mountain or somewhere.
Sometimes if I'm shooting a TLR I might have regular B&W loaded , but I'd probably have a second body loaded with either IR or colour .
I tend to use TLR's when I know I'll only want to use one film type , it's easy to just carry a separate film back than a full camera .