I agree with the two camera philosophy. I like my secondary camera to offer something the first cannot provide. My history of gear selection on big trips, with an explanation of the back-up choice:
China - Leica CL w/ 21,40,90 lenses. Secondary camera: Yashica T4 (more point-and-shootable than the CL, minimum optical sacrifice, and with a built-in flash for when required)
Peru - Hassleblad X-pan w 45 lens. Secondary cameras: Olympus XA (obvious pocketability, reliable, discreet) and Canon A1 (waterproof for a rafting trip, flash for when absolutely needed)
Colombia - Fuji GS645 w/ 75 lens. Secondary camera: Olympus XA (how I wish I'd taken more shots with the XA - the Fuji developed an undiscovered light leak and the trip was a photographic write off)
London-Mongolia - Leica M8 w/ 15,25,35,75 lenses: Secondary camera: Olympus SW 1040 (waterproof, shockproof, dustproof, in keeping with digital main camera, flash, movies)
If your primary camera satisfies 100% of all photographic opportunities, you don't have to think this way. But for me, I like to feel I'm plugging some hole left by the primary camera, even if it's used for only 1% of all shots.