People are subject to weakness. Just the act of being needlessly beligerent can put people's backs up and impede whatever persuasive powers you have. It might fire up the people that already agree with you. Angry rhetoric certainly can inspire. But when you make people hate you because you are a careless ass with a total disregard for other people's sensibilities, it can hinder people from accepting your otherwise persuasive arguments.

You might argue that anybody who won't take the right path when it has been shown him just because he so detests the arrogant asshole that showed it to him is not worth taking into consideration. On the other hand, some might say that anybody that is either too stupid or careless to take into account the realities of human behavior and emotion isn't worth listening to.

One is not an idiot or a coward just for thinking that the answer lies somewhere between the two most extreme and vocal viewpoints, just as a position is not necessarily wrong for being extreme. Not everybody comes to a point of view between two extremes just because they are disingenuous smug hypocrites.

If I was catholic missionary that went about angrily and visciously denigrating any that questioned or opposed my positions, it would only be my fault that I didn't persuade a single soul to my side. But I would probably succeed in helping others become entrenched in their opposition to me. It is stupidity.

The same can happen in political contexts as well. When, with all of your harshness and anger you paint someone as the knowing servant of satan for espousing a political position different from your own, don't be surprised when that person and others that merely heard your attacks will never see the reason of your positions.