It's an interesting phenomena. Those of us who shot film or the early DSLRs with one card slot had no backup. And once in a million there was a loss.
Fast forward to dual slots. Now that we can have backup, some of us want backup. I think there are two reasons: 1) Offering a backup mode implies that a single failure is more than a remote possibility. 2) Those of us with a heavy computer background know that, given enough time, electronics will fail.
I shoot in backup mode — with large enough cards that overflow is rarely an issue. And when I shoot — a job or just for myself — I sleep better knowing that I have two cards with the images. (Full disclosure: twice on a shoot I ran uncomfortably low on space and so blew away the backup card and turned on overflow. Ya gotta do what ya gotta do!)
And I know dozens of photogs who shoot in overflow and have never lost a frame…
Jon Kandel A New York City Nikonian and Team Member Please visit my website and critique the images!