I think that faith and rationality complete each other. In order to explain existence it is rational to use faith to support our concept of life. Although the origins of life are unknown, and to specifically identify the nature of the world's origin would be irrational, it is rational to believe that at some point the world was created. The details of the world's creation are unknown but it is rational to have faith that the world was in some way created.
The combination of faith and rationality can definitely coexist and, even can complement each other - as long as faith is not taken to the extreme. While rationality to the extreme can be detrimental in certain ways, faith to the extreme can be far more detrimental, and should be kept to a minimum.
I think that sometimes people who are strongly believing in faith lose touch of rational thought, I think people want to believe so badly in a higher power that they will convince themselves to believe in something a rational person would find to be unbelieveable and that a rational person would quesiton heavily.
Faith and rationality don't complete each other because some faiths call for believes to live beyond the rational because their god or gods is capable of operating on levels beyond rationality. Additionally, rationality calls for concrete evidence in which to base an opinion or fact, faith calls upon inherit desire to believe in otherwise means of possibility.
While a lot of people who follow a religion are just as sound as people who do not, there are a lot of people who follow and do not follow a religion that are just as irrational as each other. I do not think that faith and rationality always complete each other.