I just finished a 500 miles trip with my i3 REX without an emergency gas container. I would say unless if the gas stations are very far from each other (more than 30 miles or so) there really isn't any need for an emergency container. But I did have my car coded to allow "Hold State of Charge" and I used that all the way through.
I did end up charging the car once but only because I was at a mall and wanted to spend some time there anyway.
Having said that, if you drive for longer distances, you will eventually need some charging because even with holding the state of charge, little by little you lose some charge. This is because if you ever run our of gas, the car starts using the battery and when you eventually get some gas ans start the car again and try to hold state of charge, it will not go back up beyond where it is. So if your battery is at 40% by then, you cannot charge it back up to, say 75%. So in a very long trip, you will need to charge the battery so that you can count on it "as a backup" when you are running on gas. To avoid this to a great degree, you would need to stop and get gas just before it is completely empty.