The following article is an excellent summary of the stunning science that will flow from the SIM mission. As you can see, it will not only allow the discovery of habitable "Earths" that orbit nearby stars, but will contribute immensely to many fields of Astronomy. The review can be found at -
Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest
After reading this paper, I think that you may agree with me that even if one ignored SIM's ability to find nearby Earths, then this mission could be justified solely by its ability to advance our understanding of stars, black holes, neutron stars, and other celestial objects.