FWIW, with the current cost of gasoline, at quite low for the moment, for the price of the REx, I can go about 50,000-miles in my ICE. Then, consider that if you were to be trying to make the types of trips I use it for, maybe the first 70-miles or so would be all electric, then the REx turns on (estimated about 35mpg on average) and it would have to continue to be used nearly constantly for the rest of the duration of my trip. So, it would be getting about 15% better fuel economy while the REx was running than my ICE, but I wouldn't have to stop about every hour, or maybe more often depending on the spacing of the gas stations, and because it is always there, adding weight, I get about 7% less EV efficiency verses the BEV and don't have to pay for any motor maintenance or potential repairs (assuming I keep the car out of warranty, or in those countries that don't have the 4-year, 50K miles we get in the USA, at least once a year).
Depending on where you live, your electric may not be made with the greatest, green production methods, so add the CO2 from the electric production, the impact of manufacturing the motor, and the exhausts when running the REx, and if used on long trips with the REx required a significant amount of the time, I'm not sure the i3 is actually greener. ARound town, certainly, on a long trip where there are no CCS units, and you are not willing to spend 3-4 hours every 1-2 hours of operation (about 33% moving time), then the ICE doesn't look all that bad!
It all depends on how you intend to use the vehicle, and if you can rent or borrow, or use a backup ICE when needed. For the cost of the REx, you can rent a car for a bunch of days, too. For someone that regularly uses the thing for no more than the combined pure electric and REx use, or has ready access to CCS units, the REx looks like a better deal. In most of the USA, there are almost literally NO CCS units, and certainly few along major routes. They say that they are coming, but not everyone only drives along major routes. I used to live in Western Texas...from ElPaso to the next town was over 100-miles going east, and it was nearly 50-miles from where I lived to the next one going west. It's a different world. Longer range EVs and more capable plug-in hybrids are coming, and when one meets my needs, I'll probably sell both of my vehicles and rely on one...that wont' happen for awhile yet.
Now, someone with something like a plug-in Prius, or a Volt that both are not majorly impacted by a small fuel tank and have a larger ICE in them, those are designed to be a one-car solution, and not great at either function. The i3 excels at running around a metro area, and is barely ok on a longer trip. Depends on what you want and your situation. NEver said the REx was 'bad', just that IMHO, they added it to sell more to people that had range anxiety. Using it all of the time is not IMHO, ideal. It is a moped engine, and I'd rather not go cross-country relying on it purring away with all of its 34-hp - about the same as the original VW Bug, and if you're battery is nearly toast, about the same performance, too!
2011 535i x-drive GT, 2014 i3 BEV