Peter, I agree, just a 1 or 2 liter bottle of fuel would be just about useless unless you can see "that fuel station up ahead" and don't want to push the bike...
but 1 gallon is well worth having on board, that is 30-40 miles that I don't have to push.
Never had a need for it yet, but I came very close just south of Oarks, AR one time, I went right through our meet up point in Clarksville, AR and us guys just talked and talked.... and then we head out..... duh, I forgot to stop for fuel BEFORE our meeting point.
got a few miles further north towards Oark and happened to see the Low Fuel light bright as day.... uh oh, how long has that been burning???
luckily, there was a one pump at a tiny store, stopped and pumped in 6.7 gallons
that experience is what caused me to buy a 1 gallon Jerry can.
For me, it is critical, my medical condition will not allow me to stay out in hot sun, or in very cold temps. So, I must be able to get to the next stop. I well remember my return home from going to the east coast, temps running 97 to 101 and the humidity up in the 90% range..... I would go 1.5 to 2.0 hours and rent a hotel, or stop in a convenience store, and wait until my body cooled off....
and that was with me wearing a Phase Change cooling vest.... I can't deal with high humidity anymore.