Simple, reliable, easy to build, but not really optimum at all for a first stage. Much better for high impulse orbital transfer. Yes, nuclear electric might be more efficient, but its unlikely you're going to build a really high thrust nuclear electric design, and for lower impulse designs why not make it solar? The real advantage here though is the ease of development and deployment, you get quite a lot of bang for few bucks. The basic design of a Nerva-class system is already in the bag, it could be flying in 3-5 years if there was some impetus, and its well within the level of budget that could be realistically associated with a commercial venture.
As for not giving the exhaust velocity desired for missions to the outer solar system, there are a whole class of missions, to the inner solar system and asteroids, that would be quite feasible even using fairly primitive nuclear thermal systems. More advanced systems could be pretty interesting as well, though something of the nature of an Orion/Longshot type nuclear impulse rocket would probably make the most sense for really large manned deep space missions. We're likely at least a century away from mounting those however, so its not exactly something that relates to SPS.