I get things done in all those cases on the setup I mentioned. Obviously if you're taking 8-hour flights then you need that much battery, but is that a common case?
It's a very common case that you have a hard time finding a socket in some places. It happens a lot to me that many others are working on their laptops and all sockets are taken, or even that there are no sockets at all in some trains.
Introduces an extra dependency.