I heard a great one yesterday. A friend of mine told me,
"There is no way that evolution is not true...just look at my daughter, she is nothing like her fat, loud mother."
When asked what evidence this gave toward evolution, my friend answered, "I don't know, but hadn't we all better hope evolution is true? Otherwise, we'll keep producing mother-in-laws and ex-wives. Now who needs that?"
If you have to have a reason to believe something is true, I guess maybe that's as good a reason as any. :)