OK, rather than my words, I came across this...
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The NRC report Astronomy and Astrophysics in the New Millennium calls for building of the Terrestrial Planet Finder only after demonstrating that Earth-size planets are common. Quoting from the report about the TPF:

"The committee's recommendation of this mission [TPF] is predicated on the assumptions that TPF will revolutionize major areas of planetary and non-planetary science, and that prior to the start of TPF, ground- and space-based searches will confirm the expectation that terrestrial planets are common around solar-type stars." (pp. 12 and 39)

"In addition, to ensure that TPF reaches its full scientific potential, it is important to determine prior to the start of the mission that it is likely that there will be an adequate number of Earth-sized planets for TPF to study." (p. 112)

The Kepler Mission does determine if terrestrial planets are common as well as, characterizes these planetary systems and determines their distribution.
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So, the NRC stated precisely what I'd said, build TPF ONLY if Kepler, et. al. show that the results would be worthwhile. Like Mark, I would build TPF anyway just to research planets per se, but my point is that the politics of science will not allow TPF to fly if we find that Earthlike planets are truly rare, i.e. likely ZERO within 50 LY.

-philw (it has little to do with who is the NASA admin)