I'm reading Sam Harris's End of Faith with my Sunday School Class at Chalice Christian Church. I think it is fair to say that Sam Harris defines faith as irrationality. This allows him to do a couple of things. One, in Chapter 3 of his book he can make the claim that Nazism is essentially a religion. Two, in Chapter 1 and elsewhere, he can claim that rational adherents to a religion are not really religious.
What I take from this is that rational religious folks need to work hard to make other rational people understand the value of faith to a rational person. The shortest answer I have for this is that the human condition involves a spiritual/soulful something that is neither defined by our intellect or our physicality. Religion and faith help nurture and grow that aspect of our humanity. Developing our sense of compassion and empathy, feeding our better angles, and sharing our progress in these efforts with others is important and entirely rational.
The other thing I take from Harris's book is how tremendously dangerous irrational beliefs are. Irrational fears whether brought to us by religious leaders, political leaders, or news moguls all seeking to consolidate power are dangerous. Harris's book is really a critique of the frequent failures of humanity.