Seems plausible, although I suppose it depends on what you want to do once you're in space and where you want to go.

If you could get a mass driver system to work very reliably (more reliably than chemical rockets), I could see it being useful to deliver the critical assemblies of RTGs into orbit. Plutonium is pretty resistant to high acceleration. One of the main objections to putting a lot of RTGs in space (or even better, actual nuclear reactors) is what happens during a launch failure. A system that doesn't involve any fuel, and thus won't blow up and scatter the contents around, or require tons of containment, might be useful.

Nuclear reactors have a pretty nice energy-density profile, particularly if what you want is long-duration energy for interplanetary missions.

Still hard to get around the capex problem though.