I've heard a number of writers warn against putting Strawmen in novels. If you're not familiar with what a Strawman Character is take a moment to read up on it. Finished? Okay.
I understand why they're not the ideal but I didn't fully understand how they could damage a story until I saw Catching Fire a couple nights ago. Now I will preface this with the disclaimer that I haven't read the books. There aren't enough hours in the day to read everything I'd like to so I have no idea whether the character in question was as bad in the books as he was in the movie. That aside, he was glaring aberration that should never have been put on the page. So out of all the personalities on screen, which one did I want to direct the holy flamethrower of death upon? Commander Thread.
Yes, I know that we're meant to hate him and that goal certainly was accomplished in the film. However, that was no excuse for the screenwriters to make him an ignorant grunt that doesn't care whose toes he tromped on. It's counterproductive. His character has a given job to do, but the speed and blind efficacy in which he does said job hurts villain's cause -- the very person he's working for. Instead of being a character that added depth and conflict to the plot, he detracted from it and made the villain (and the writers as well) look like an idiot for not foreseeing the outcome. In some ways it also undermines the hero/heroine. Instead of their choices being difficult and their plight harrowing it became a "well it's about time someone did something about that mess" scenario.
Not all strawmen are this detrimental. Thread's character was so over exaggerated that his affect was more widespread but it does make it pretty clear why the trope is justly frowned upon. In some ways I feel bad that it took such an overt example for me to understand that. Oh well. At least I can count it as a lesson well learned.