It's the journey, not the destination
JIMO is not about exploring Jupiter. JIMO is an excuse to build Prometheus. What's the closest world where you need to have a nuclear electric propulsion system if you want to get there quickly? Answer: Jupiter.
JIMO is really an engineering prototype and nothing more (think DS-1). I should add that the nuclear electric propulsion system to be used by JIMO is such a quantum leap over earlier concepts that an engineering prototype is necessary.
Gary