The problem with rational debate is that people aren't rationale :)

Christians are too often convinced that

1) What their parents taught them is, in fact, Biblical; and/or
2) What their Sunday School teacher taugh them is Biblical; and/or
3) Entertaining that they may be wrong is akin to a "lack of faith" in God.

More importantly, American christians are comfortable. Their worldview is an old shoe with which they just can't part. Challenging them may force them to love their neighbor.