Any opposition to a religion helps in one regard: it lets people know that there’s a diversity of views.
People often have a tendency to assume that others agree with them until they have reason to think otherwise. I think this is behind the stereotype of the “angry atheist:” many people just assume that a pleasant, quiet person must be religious like them, so the only atheists they acknowledge are the loud ones.
Expressing disagreement - tactfully or not, fairly or not, logically laid out or not - will send the message that you disagree. This can be useful by itself. If enough people do it, it can make the person realize that their position isn’t as popular as they thought.
Expressing disagreement forcefully in a way that makes your offended clear communicates to the person that at least some people consider their beliefs offensive. If enough people do it, then it’s harder for those objectors to all be dismissed as cranks.
Sometimes this is enough. If a person’s religion tells them, say, that they’re messengers of truth that the Holy Spirit will use to communicate the Good News, then failure to convert people could undermine their faith by itself. If enough people flip the bird at the protestors picketing the hospital with photos of dismembered fetuses, maybe some of the more thoughtful protestors will ask themselves “what is it about my message that people find so offensive?”
Yes, there are other ways that a person can interpret disagreement and offense, and many of them end up reinforcing the person’s faith, but that’s not the only goal.
If we can get people from a harmful religion to say to themselves “this town is beyond hope - let’s not even try to proselytize here” or “we know that elected official won’t be sympathetic to us, so let’s not even try to lobby him,” then those are wins, too.
I have nothing at all against convincing adherents to abandon a harmful religion - if you can do it, go for it. But it’s also a perfectly valid strategy to try to just contain the harm of a harmful religion, and the tactics for that strategy don’t always involve changing the minds of the adherents.