I see people bring this up a lot online but on a lot of TV and features I work on we do in fact have backup camera bodies... Now when I'm mixing an interview or "branded content" (low budget commercial) there seems to be a little less contingency planning, probably because the jobs are shorter in duration. On a long film or TV show, you're almost guaranteed to have a camera body fail at some point in which case you pop the next one on there.
As far as rolling a backup, I've considered a Lectro SPDR or a Zaxcom ZFR300 for a backup recording of my mono mix in the bag, but haven't seen the need to invest yet for my mixing work. Now that I'm thinking of it, I should at least rig a Zoom H1n in my bag for this as it does two channels. I personally think you get the most use out of that on TV or films. When the cost of individual takes starts increasing (large crews, stunt work, lots of rigging work, you know the stuff...) I've seen a mono track backup recording save productions a lot of money firsthand in reshoots.
For now whenever I'm renting equipment to a production I always keep a backup recorder close by in the kit. I am in the rental game after all, and rental houses arrange a replacement when one of their items goes down in the field.