When I was considering what adventure bike to get, one the things I focused on was range. The 310GS plus one extra gallon of gas (~6 lbs) is on par with the R1250GS in terms of range, but it would take roughly three extra gallons of gas (~18 lbs) to be on par with the R1250GSA. Because of this, when I choose the 310GS, I also bought 4 1L MSR fuel bottles with the intent to adjust how much I carry based on how easy gas is to get along my planned route.
On my 7,000 mile North Carolina/Missouri/Nova Scotia/NEBDR trip this last June, I carried two 1L MSR bottles of fuel. The only time I actually used one of them was to help a Harley rider who ran out of fuel 1-1/2 miles short of his intended fuel stop. I'm planning on more BDRs next season and I will continue to carry at least 2L of extra fuel, but may up that to 4L for some of the western BDRs.