Knowing lots of facts absolutely matters in the real world. Having a good theoretical framework to organize them and logic (formal, informal, probablistic, or otherwise) to manipulate them help too - but not without real facts. Facts alone are worthless, you need good judgement too, but you cannot function at all without facts.
"How emotionally entangled are you with your point of view? Test yourself - defend an opposing view, believing your life depends upon it."
-- Marc Stiegler, David's Sling
This is really only useful for non-factual arguments, where your emotions are more likely to interfere. You can prove ANYTHING if you can ignore disconfirming evidence. It reminds me of a point in decision making - almost all of our choices are over-determined; that is, there are always good reasons to do almost anything we may want to; to make a good decision we also have to balance the reasons not to choose particular things.
"What are the facts? Again and again and again - what are the facts? Shun wishful thinking, ignore divine revelation, forget what "the stars foretell," avoid opinion, care not what the neighbors think, never mind the unguessable "verdict of history" -- what are the facts and to how many decimal places? You pilot always into an unknown future; facts are your single clue. Get the facts!"
-- Robert A Heinlein, Notebooks of Lazarus Long and Time Enough for Love
One must be ready to discard his own ideas and theories if new facts overturn them. I think knowing lots of facts is the simplest way to notice and correct for your own biases. You are more likely to notice a contradiction between what you have just decided and a previously known fact than you are to directly notice a bias in your thinking; even after setting the decision aside and coming back to it you are still more likely to notice a contradiction than a bias.