Mainly useful inside of Mars orbit. Otherwise the solar panel size needs to be way too large to be practical.
This probably came up/out now in response to the news that the DOE has no more Plutonium or funding to make/process any more for RTG's to power missions for the time being. Even Mars gear needs Pu for keeping stuff like rovers warm, even stuff on the Moon could use it for thermal management during the 2 week long night.
Of course we could just build actual Uranium reactors then... The Europa orbiters/landers/drillers/subs will need them anyway right?
Although the magsail and ED tether might have some use in/around Jovian space maybe? Once the probe got there?