A reasonable manufacturer looks at its bottom line and doesn't waste his engineering hours and BOM costs (double the flash memory!) on silly features like that. Either that or there are many unreasonable manufacturers out there.
- How much do you think the NOR flash costs inside this cable?
- How big do you think the firmware actually is?
- 16MB Winbond NOR flash is like $0.70 with no price breaks (for me, I am not Apple making 1 million units) - there's no way they need 1MB for two slots, much less 16MB.
- These cables retail for $49.
Two firmware boot slots is standard practice because it is essentially free to provide in any higher-end consumer electronic device: especially one that is explicitly designed to be unplugged easily.
How many support claims are saved on this $49 cable because mid-update bricking does not occur?
In practice pretty much every embedded device I've worked on that has supported in-field firmware updates has had either double buffering and/or a recovery mode to limit the possibility of bricking.
For the products where every last penny really needs to get squeezed out of the BOM (toys, low-end appliances, ...) the ability to do in-field firmware upgrades itself is IME often one of the first things to go.
BTW, they don't have to double the flash memory. They just have to provide enough extra memory for a fallback updater that the boot code can load if the main firmware is corrupt. All the fallback updater has to handle is telling the computer it needs firmware and then installing the firmware the computer provides.