One problem with nuclear reactors on space craft is what happens if/when the craft re-enters the atomosphere? How do you stop the reactor from becoming a high-altitude dirty bomb?
If the fissile material could be acquired by asteroid mining, then it would never need to pass through the Earth's atmosphere. I know that still sounds way out there, but I think that uranium or thorium would actually be a great material for kicking off asteroid mining. A small amount will pay off in a big way, it is a very dangerous thing for humans to mine on Earth, and, like the parent mentioned, we really don't want to fly it around where it can fall on us.