http://www.amazon.com/Thank-You-Arguing-Aristotle-Persuasion...
There are two main reasons that fallacies are bad. The first is that they're not rigorous, which is why parent mentions pure mathematics. Perfect logic matters, sometimes. Strong philosophical positions are like this, too: the kind that takes you years to develop and stabilize. But it's actually pretty rare for anyone to spend much time here.
The second is that fallacies are meant to contort the mind. Which isn't a bad thing, but if you're not equally equipped in logic and an understanding of fallacies yourself, deft usage of fallacy will run circles around you. While virtually all thought experiments are fallacious by nature, if you can't figure out where the boundary between allegory and reality are, you're going to have a problem.
