To put it another way...the Volt and Prius both have adequate ICE motors and fuel tanks to be used exclusively, if that is required to get where you want. The i3 REx, with its small gas tank paired with a small ICE, and serial hybrid configuration, is not well suited for longer range. When I do take a trip, it's usually in the 4-800 mile range, and not stopping at nearly every rest stop on the Interstate as would be required on the REx, is a big advantage. My ICE can often get over 500-miles on a tank, so I could go all the way without stopping if I didn't need a stop for some other reason. Plus, it gives me the opportunity to buy fuel where it is cheaper, rather than right on the interstate or just off of it where the prices tend to be higher. Someone living on say, Hawaii, it's really hard to need more range than real-estate available...that's not the case on the mainland or in most other places.
The i8, on the other hand, has an adequate ICE, can recharge its own on-board battery pack, and a decent sized fuel tank. I'm sort of hoping that the i5 will be offered in a similar configuration, and it will then replace my two vehicles pretty much without compromise...plenty of range for day-to-day operations on pure EV mode, but more than enough range to cruise forever without having to search for charging stations that, at least where I live, are not particularly numerous or convenient. Plus, using one on some networks, could end up costing more than the gasoline to go similar distances. Neither the Volt nor the Prius appeal to my senses, nor, do I fit particularly well in either.
2011 535i x-drive GT, 2014 i3 BEV