Such intense emotions can show a lack of regard of differing views, and weaken any argument you may try to make. Your ideas may come off as simplistic and one-sided, and your tone overly forceful or abrasive. Nobody wants to be told what to think, or treated as if they are too stupid to make their own minds up. Such a tone is likely to make a reader less sympathetic toward your cause or views. Anger can also lead to spontaneous and unplanned writing, which is bound to drift off topic and waste your (and the reader's) time.
