When we're arguing against bad ideas, especially when we're doing so in a place where our ideas are predominant, we should be arguing against the strongest possible version of the ideas. If you get used to arguing with a strawman version of the argument, when faced with a smart person who actually has those ideas you may get entirely trampled and their ideas appear to onlookers as the better ones. This is applying the principle of charity, but it's also a way to improve the strength of our own arguments.