Some of the engineers involved in molten salt reactors say "come for the thorium, stay for the reactor." Liquid fuels offer several benefits. Removing fission products continuously helps you achieve high burnup. There's no fuel fabrication cost. And if the electricity cuts off, a frozen plug melts and all the fuel dumps into a passive cooling tank, with little worry about decay heat since you've been removing those fission products.

The neutron economy is arguably an advantage from a proliferation perspective, since you can't remove much fissile without the reactor shutting down.

Still, a good fast reactor like the IFR has its own charms. The metal fuel is easy to fabricate on-site, and they tested electricity cutoff and it quietly shut down just fine, due to the physics of the fuel and coolant. And the mixed plutonium isotopes produced by the reactor are very difficult to refine into weapons-grade.

One advantage of the molten salt reactor is that there's nothing in the reactor that can drive any kind of chemical explosion, like the hydrogen that blew at Fukushima. The IFR uses molten sodium, which is pretty reactive with air and water. That might not be too terrible to deal with but it does drive up the cost.

Another advantage of the IFR though is that we've got a production-ready design ready to go.