Since the REX engine is a component of a "serial" hybrid, making the REX engine available as a dealer-installable upgrade/option would be beneficial for the following reasons:
1. Environmentally beneficial:
(a) Many inbuilt-REX purchasers may use it only occassionally, if at all. So, lugging around ~300 lbs at all other times isn't good for the environment, along with construction costs and minimum maintenance required for the REX engine.
(b) Many current ICE owners who balk at the idea of EVs might reconsider if they are assured that an ICE can be added by the dealership, as their range/flexibility needs change.
2. This would go well with the modular concept that has been used with the i3 design. Along with battery upgrades, one won't be boxed in with the initial range/inflexibility of the car.