SLS is definitely an expensive option, however I'm not sure the cost:benefit trade-off is as poor as you make it sound.

http://spaceflightnow.com/2015/03/10/europa-clipper-concept-...

"The mission concept review board in September concluded the SLS launch option was 'far superior' to other alternatives because it would allow the probe to fly directly to Jupiter in less than two years. Launching on a smaller rocket would require gravity assist flybys of Venus and Earth, adding three-and-a-half years to the voyage."

I think SLS makes a lot of sense for outer planet missions - at least until commercial super heavy lift options are available.