You could simply duplicate your A-cam, of course, but your budget may not stretch that far. Besides, running a B-cam with a different feature set can enable you to cover more angles, more creatively. There might be other practical considerations too. You might need a camera that can shoot underwater or at a very high frame rate, for example, or you might simply want a more compact camera that will fit into a tight space and can be handheld more easily.
Working with multiple cameras can bring additional complications. With mismatched A- and B-cams, you may need to spend a significant period in post-production to ensure your footage has a cohesive look. Different sensors, recording formats, data rates and colour sampling can all lead to extended working times.
"Anything you can do to make the images look similar to start with is only going to benefit you when it comes to post," advises Paul Atkinson, Pro Video Product Specialist at Canon Europe. "It'll mean you spend less time transcoding and applying different grades to footage from different cameras."