Every now and again, I see accusations of "bad faith". How important is this as a criticism? When someone acts in "bad faith", what, exactly, are they doing? should they stop? and what should we do when we come to suspect it of a potential interlocutor? In answering these questions, I'll also argue that we should stop using the phrase at all.
When a person acts in bad faith, they do so usually in making moves in some conversation or broader discourse. They might do so through an isolated speech act, say a particular assertion, or through a pattern of speech behavior. Often what seems to be meant is that a person is insincere, where that means, roughly, they don't believe what they're saying. Trolls belong in this category, as do "devil's advocates". There are other varieties of bad faith it helps to be aware of. The first is when a person doesn't seem to be open to arguing for their views, as opposed to (if successful) manipulating their audience into coming to have them. The second is when a person is willing to argue but not willing to engage; here it helps to think of a debater who hopes to "score points" but isn't considering opposing views as possibly true, or opposing reasons as possibly important.
Thus, we have these characters:
There will be no uniform answer to whether it is bad to be one of these people. Devil's advocacy is sometimes perfectly appropriate, even when the devil's advocate fails to disclose that that's what they're doing. Trolling can itself be fine: sometimes levity adds an important dimension to conversation. But both can of course be very bad, when they are distracting or uselessly offensive. In extreme cases they can render conversations complete wastes of time because of the distraction they cause. The rhetorician and debater are, I think, more morally marginal cases.
Start with the rhetorician. I gave them the name in honor of Plato's Gorgias, which is devoted, at least in large part, to displaying the badness---moral, political, and maybe even spiritual---involved in psychologically manipulating people's beliefs. There are many criticisms one can make of such manipulation, of course: that it violates people's autonomy (as manipulation often), being in the best case a kind of paternalism; that it degrades discourses, by making people believe things that are often unreasonable; and that it insults people, by disrespecting their reason. I think those are more or less right. (Plato does have some of those criticisms, by the way, but I think he is as much concerned with the harm it does to the person doing the manipulating, but that's a topic for a different post.) And yet even recognizing all of those real harms, there might be cases in which such manipulation is important, as when an oppressed group skeptical of its own political power comes to believe in itself by means of stirring speeches. There is a place for rhetoric, even if we must be careful about its use.
The debater is, to my mind, a different story entirely. There is no use of such a person I have been able to discover. They appeal to reasons, but are not open to contrary ones. They are typically not even amusing, and their lack of openness makes them unsuited to inquiry, in contrast to the devil's advocate. They might empower people, but they could do so just as well by being open to contrary reasons. We should all avoid being debaters.
The question, then, of whether someone who is acting in bad faith should stop will not have a uniform answer, either. Maybe they should, and maybe they shouldn't. If they're a debater they---we---should. But it's important, I think, to understand that bad faith has a discourse function. Sometimes the particular function ought not to be performed in the given circumstances. But we need to pay attention to these circumstances, and what we mean when we say a person is acting in bad faith, before we can be confident that such a person should change their behavior. The problem with the expression "bad faith" is that it makes a heterogenous category seem homogenous, and together with its pejorative connotations, we can prevent important discourse functions from being performed. (This is in some ways similar to Jessica Wilson's criticisms of "big-g Grounding": be more specific about what you mean!) Were I in charge of the use of language, I would strike "bad faith" from people's vocabularies; it obscures much more often than it illuminates.
What, then, should we do in response to someone who seems to us to be acting in bad faith? Again, there will be no uniform answer, but perhaps there will be something a little more uniform than earlier in the discussion. If it's a troll---a good one---then the response seems to me to be: laugh! And engage, if they're saying something worth engaging. This does often happen, by the way; mockery and sarcasm may sometimes frustrate, but there is often a thought behind them worth responding to. This leads to a broader point: just engage (at least before it gets repetitive!). Even debaters make points worth responding to. It sometimes protects ourselves from the points that the insincere and/or the closed-minded make to dismiss them as insincere and/or closed-minded. Even so, we shouldn't care who makes the given points, or whether they are behaving perfectly in the conversation; we should turn even bad behavior to profit, but we ensure that we don't do this when we don't think about what the person says and possibly learn from it. Using "bad faith" as a criticism encourages us to not do this, but that, in its own way, makes debaters of us; we will present our own reasons, but not accept the reasons of others. This profits no one. It doesn't even disincentivize the people acting in bad faith from doing so in the future, because they will then think that their interlocutor is themselves a debater, and themselves acting in bad faith. This is an ugly cycle, and leads to an unhealthy obsession with meta-discourse. (I realize the irony in my saying that while engaging in meta-discourse.)
So, stop accusing one another of acting in bad faith, and if the person is in fact saying something new or interesting, just respond! And don't spend much time puzzling out their motives or broader discourse-dispositions in deciding whether to respond. That, itself, pollutes the discourse.