Maybe the most common concern of someone coming from an ICE to an EV is range and what to do if you suddenly run out in the middle of nowhere. BMW foresaw that concern and offers the i3 with the range extender for those that needed the assurance to relive anxiety. I'm sure that they would have preferred a simpler BEV vehicle, but were realistic...people fear the unknown whether it is realistic or not.
In my case, I was confident that the REx was an added cost and unnecessary for my use pattern and would have possibly reconsidered the vehicle entirely if I'd been forced into buying one with it.
If your use pattern requires longer ranges than you can reliably achieve with the BEV, the REx version MIGHT be a viable solution. Some people are totally satisfied with smaller vehicles, but they do present some issues on longer trips when carrying more people and/or luggage. IF you accept the limitations of the i3 BEV or REx, it may be a great choice. I was pleased to trade my daily driver ICE for an i3 for my normal use, but am pleased I kept it for those situations where it excels.
As BMW adds more vehicles to their ix family, you will see designs that address some of these points. Some rumors are that the i5 will be a fuel cell based vehicle based on their agreement with Toyota. Longer range is a given with that type of vehicle, but it probably has an even bigger problem with infrastructure than BEVs. There has been some recent advances in small-scale hydrogen fueling station tech such that it can be produced on-site economically, on a footprint of a typical gas station. You're stuck with the chicken or egg conundrum, though, they won't get built if there aren't users, and there won't be many users unless they can be fueled...sort of where we are with EVSE and CCS units, but it's much easier to get one of those installed than an explosive hydrogen plant! And, shipping the stuff is not easy, either - it is hard to contain the smallest molecule known to man, compressing it to liquid takes a LOT of power (not counting that that it takes to extract it from whatever the source is), and the extreme cold makes most materials quite brittle adding to the containment issues. Throw in that it burns without color and is tasteless and colorless, and maybe you'll begin to see H2 is not without issues as a fuel for vehicles. In the right combinations, H2 doesn't even require a spark to ignite it.
