Gravity Slam Dancing
The problem arises from the gravitational dance of the binary stars. As they dance tightly around a common center of gravity, they find themselves moving closer to each other, always facing each other. This is not a sweet, romantic dance. The stars spin rapidly, creating massive magnetic fields and intense solar winds. But worse yet is that the intense solar winds slow the stars down, pulling them closer to each other. As stars dance closer, their gravitational effects on the planets orbiting them change - creating chaos and a great likelihood for collisions between planets, asteroids, and comets.
There Goes the Neighborhood
This chaos does not bode well for life. While it may be argued that some chaos is good as it may add to evolutionary pressure, too much chaos is not good - especially if that chaos means your planet colliding with another, or being pummeled by one too many extinction event asteroids. And even if your planet does not collide with another, it may change orbit, moving out of the habitable zone. Either way, there goes the life-giving neighborhood.
This is not to say life in a tight binary star system is impossible, or that high order sentient life cannot evolve and survive; but it does mean that such a system is not the top candidate to target in a search for life.
Clavin, Whitney. "Pulverized Planet Dust May Lie Around Double Stars." Jet Propulsion Laboratory. NASA. 23 August 2010. Web. 25 August 2010.