I see one use case: leaving the car for extended periods of time unplugged, say at an airport parking lot.

Last year I had to leave my car unused for three months, after which both the 12V battery and the hybrid battery were completely discharged.

Had there been a solar panel on the roof this wouldn't be an issue.

Nowadays most of the cost of solar is installation, so I don't think such a panel would be very expensive.

The 860W panel on the Prius Plug-In is a €2000 option, so it's around twice the price of a rooftop installation, but they're using 34% efficient, monocrystalline cells, so most of the expense comes from that.