The phone also charges with your AC charger though, and likely faster. Your gain is merely having to get just a cable out, instead of cable + AC adapter.

And if you're worried about a shortage of outlets, the savvy traveler thing to do is carry a multi-outlet tap and facilitate sharing, which also works at 120/240 and not 5V.

Some places don't provide a power socket I can plug an adapter into. Just the USB socket. Sometimes I'm carrying a phone and a slim cable I don't mind putting in my pocket, but I really don't want to put an AC adapter in my pocket and carry that around with me. Sometimes the AC adapter is in my checked luggage rather than in my pocket. Sometimes my AC adapter is made for a different kind of socket and I don't have a travel adapter for it. Sometimes I don't have a cable or an adapter, but someone lets me use their cable for five minutes. There are a lot more combinations in life than carrying a bag with an AC adapter inside that fits the local plug shape.

And if you're worried about a shortage of outlets the savvy traveler thing to do is carry a multi-outlet tap and facilitate sharing

Carry MORE things? I'm not worried about a shortage of outlets; I just use the USB slot.

I guess you've never experienced a watering hole of people at the only power outlet around? And aren't foreseeing being somewhere without USB jacks but with regular receptacles?

There is a whole bunch of path-dependence in what you've said. I formed my approach a decade ago, primarily in the context of traveling, and haven't seen a benefit to revisiting it.

But surely by the time one is talking about buying special cables with no data lines, but still needing to do power negotiation somehow, maybe it's time to question some recent assumptions!