Bridging 12 V and 48 V in dual-battery automotive systems

I read that article when it came out, but am puzzled about its usefulness. The LM5170 is a very complicated bidirectional buck-boost controller, meant SFAICT to support current flow between two voltage systems, arguably two battery systems. But why have two batteries?

Why not just the 48V system, with a serious buck converter for 12 volts? At these power levels, it'd be a synchronous DC-DC converter.

Running a synchronous converter at a fixed duty cycle gives you the desired output voltage, as a ratio of the source voltage, good enough, and also steps 12V current back up to 48V, in case of load dumps, etc. I've made these to work at high currents (driving large magnetic fields), and was surprised at how well they work. If anybody wants to play with this, using a bare PCB, ask for RIS-671, PWR-671. My design uses an IR2085 to set the duty cycle, and drive a pair of big MOSFETs of your choice.

Dunno- economics first, common sense second.