The BMW i3's range extender provides for a very different mix of electricity vs. gasoline. The electric range will be 80-100 miles, but the range extender is a much smaller engine than in the Volt. Perhaps it will be a two-cylinder motorcycle engine with 650 cc displacement. BMW has not given the exact data of the final version.
Obviously, the power output of such a tiny gasoline engine may be insufficient to recharge the battery fast enough to enable the battery to provide highway-speed propulsion. Perhaps it will only allow you to drive at less than 40 mph, or perhaps you won't be able to drive at all while this little engine builds up juice for the battery. You may have to pull over and wait 15 minutes to give you that extra 10 miles worth of power. BMW has just not provided these specifications yet.
Equally debilitating will be that the gasoline tank will probably not be larger than 2.5 gallons, in order to fit a new California government requirement for this particular class of plug-in hybrid car. Yes, I kid you not: It's California's equivalent of New York City Mayor Bloomberg's large soda ban. The gasoline tank will not allow you to drive more than 100 miles on one gasoline tank. Perhaps the civilized, i.e. non-California, world will get a version of the BMW i3 with a larger gasoline tank.
Anyway, you see the difference here: The BMW will give you 80-100 miles on electric power, and then another 80-100 miles on gasoline. This ensures that you will never get stuck, having miscalculated the electric range, failed to charge somewhere, or simply had an emergency occur. However, this 80-100 mile worth of gasoline range may come with a significant degree of inconvenience, as described above. If so, it'll be there only for emergencies -- not for everyday or long-distance practical use, as in the Chevrolet Volt.
Many people want an electric car, and they are fairly confident that on 99% of days they will not drive more than 40-80 miles. However, 99% is not the same as 100%, and they need a reserve that can ensure they will never get stuck and that can handle the unexpected change of plan.
The target BMW i3 consumers don't aim to take this car on a road trip from Los Angeles to San Francisco. They plan on driving it only inside any
of these metropolitan areas. There are many who fit this description, and for these people the BMW i3 with range extender could be the right tool.