Stopping by a gas station every 100 miles isn't a huge issue for road trips. I don't know of a 100 mile stretch of road in europe where there are no gas stations. (It might be an issue in Alaska and the like, though.)
It would be completely inexplicable if BMW had decided to implement the REX in such a way that road trips were impossible, as the reverse is completely trivial. If the car comes with a "charge hold" button like the Volt, the driver can decide when to use the gas engine and when to use the battery. If you press the "charge hold" button at 50% SOC, the remaining SOC would be more than sufficient to power all 170 electric hp, and 34 hp is more than sufficient to sustain highway-speeds.
If the i3 doesn't have a "charge hold" button like the Volt, BMW seriously screwed up. I can see the advertizing already: "All the utility of a Leaf and cost/complexity of a Volt!"