Sure it has a higher specific thrust, but they still haven't solved the problem of getting enough electricity to run it on a spacecraft. You can't keep a fission reactor cool with nothing but black body radiation from the surface of the craft unless it has a huge surface area. If they shoot a huge laser at the craft, converting that to electricity is still going to have low efficiency, so you still have the problem of cooling. Obviously the high weight-to-power ratio of any kind of chemical battery would defeat the purpose and probably result in an overall system no better than H2/O2 rockets. The best they've got is betavoltaics and photovoltaics. 500 square meters of solar panels to produce 50KW at mars' distance.