Indeed, that is true. I just had the same thing happen to me. I rode from Tallinn to Croatia to Italy to Austria to Tallinn during the past 2 weeks (need to make another post about that) and my USB outlet failed in the Slovenian mountains. I went to a gas station, bought s car charger and long wire and built myself a makeshift charger directly from the battery.
Anyway, my idea was that the final solution to this should be a real quality IP-rated USB socket, mine was a $3 from AliExpress and failed due to heat and/or vibration. Having a backup 12V would have indeed saved me a few $ and a hour of time.