Why do Christians and rationalists discount the paranormal so often?
I think the problems often stem from human nature with a mixture of pride and fear and that as a rule we prefer to believe what we prefer to be true.
I have seen rational believers and non-believers argue their views with religious zeal often citing the need for, or requiring the listener to exercise faith and "Faith" is a word often banded about as a cover all for those too lazy or too scared to dig deeper into their own life "support" systems, I don't believe that faith is blind acceptance without question, I believe the Bible's definition in Hebrews 11:1 that "Faith is the assured expectation of things hoped for, the evident demonstration of reality though not beheld" I like that verse because it uses words such as "assured" and "evident" which to me says that whatever a person chooses to believe they should have assurance and evidence to support their faith sufficient for them to be comfortable with it in open debate.
So why have I answered your question with a paragraph about faith? Its because I think that any belief system we have relies upon it, whether it be a belief in a god, Christian or otherwise, or a non-belief such as an atheist may have about evolution or the views of a medium on the nature of the paranormal. As humans understand more about the universe and the nature of our existence in it what actually is a "proven fact" will change and I feel its important to have a faith robust enough to help us be at one with it.
Why are these issues "discounted" often in surprisingly irrational heated debate? Because the anger of the implacable opinion is directed more at the doubt within than the enemy without.