To some of us, the most important reason for carrying two bodies at least, is having two kinds of film available at the same time, or two kinds of development even if using the the same film... Two bodies, is a compromise already, but two is a much better solution than only one...
For example, wild direct sun requires a really different treatment unless the professional wants to lose all zones in shades and decides to provide an overall look that doesn't remind of a sunny feeling, where we see with total clarity the zones in the shades... Apart from the most common soft light scenes and the wild contrast ones, the third situation that's nice to have at hand, is the fast film/development/body for low light.
The best way to treat 35mm film is obviously the same way as sheet film is treated... Using one single body and one single film and development for everything isn't better in any way: it's easier, yes, and lighter, yes, and for sure it implies compromises... But better, never.
Cheers,
Juan