Demonstrating large-scale solar electric propulsion (SEP) systems.
This is one of NASA’s main interests in the ARM mission — in the land of expensive launch vehicles, very high specific impulse propulsion, like you can get with SEPs, can make many missions a lot more affordable. Even with low-cost Earth-to-orbit transportation, SEPs probably make sense for a wide range of missions. Demonstrating the ability to use large-scale SEPs for tugging huge objects in heliocentric space, performing precision injection maneuvers, etc., might be very useful.
The goal of high-isp interplanetary transit is indeed what I would consider an enabling technology for colonizing our solar system. Without it, there just isn’t any way to get the mass of hardware and logistical items where you need them to survive in extraterrestrial locations, at least in near-term scouting missions. When ISRU can be reliably counted on at each location we colonize, that picture changes at least theoretically, for that local colony. However, we end up in the same situation again as we hop from location to location in our system, and we must be able to take our needs with us to each new place.
Now for crewed missions in general, we need high-isp capability to keep trip times low, and avoid the years-long travels that we have long taken as a given. SEP propulsion systems don’t address this directly, being ill-suited for long-term use in transporting crews and all the large systems they require. Developing SEP to a higher degree of sophistication does bolster the mission design for unmanned cargo systems, of course, and that would reduce the burden on crewed missions somewhat by offloading some of the logistical burden. Essentially, the SEP systems for colonization efforts would be relatively slow-moving uncrewed cargo ships sent ahead for later rendezvous with much faster crew carrier spacecraft. For the carriers, other bootstrapping rationales would be needed to co-develop technology like VASIRM, where the power density is much higher than that of solar-powered ion engines.