It is worse to be irresolute than to be wrong.
This paraphrased wisdom, yet another gem from Strunk and White's The Elements of Style, is a textbook example of the sheer arrogance--and truth--of William Strunk's philosophies of writing.
Every writer must understand that something said decisively in a clear and confident voice sounds far more "right" than something said indecisively and without confidence--regardless of the facts and evidence.
Yes, you need facts, figures and evidence to back up your assertions. But once you've laid down a solid foundation for your argument, say what you want to say forcefully. And at all costs avoid namby-pamby, qualified language.
If you cannot write resolutely, don't write.