I hear this criticism from time to time, leveled at a range of targets, from the recent "new" atheists, to people arguing for the validity of evolution in the face of creationist claims, to right here at JWD and criticism of Jehovah's Witnesses and other forms of fundamentalism.
"You are just wasting your time, preaching to the converted", and "Believers are not going to be swayed by such criticism of their faith, so why bother?"
I'm not sure I had a coherent response to this before, but I do now in the form of this concise and insightful video from a youtube favorite of mine:
http://www.youtube.com/watch?v=H57Z0yE3Qgw
-inkling