This, my friends, is the Fallacy Fallacy. It is the act of saying that someone who makes a fallacious argument for a claim must be wrong about the claim.
I noticed that! My own way of dealing with it is to interpret a logically-fallacious argument as an opinion rather than a falsity. Case in point: someone who uses the "slippery slope fallacy" is really expressing an opinion that a slippery-slope situation exists. He or she is also making a forecast based on that opinion. The forecast - eg., allowing same-sex marriage would encourage a push to legalize polygamy - might be wrong, but it might be right. Since it's based upon an opinion, it should be treated skeptically - but it should not be dismissed out of hand.
For what it's worth, the favorite fallacy of the "Wiki-philosophers and Google-logicians" - in my opinion - is the Genetic Fallacy, dressed up in ostensibly practical-minded clothes. Cases in point:
- "Did the oil companies pay you to say that?"
- "Only Fox News would publish that rot."
- "Wow! The Gospel According To Rush Limbaugh!"
- "Did you know that Sarah Palin said the same thing?"
...and so on. I'm sure someone can adduce similar examples of the genetic fallacy from the other side of the aisle.