How about it raises the costs of coordinating a conspiracy - it signals to other protest groups that if someone starts suggesting violence, you should decline b

How about it raises the costs of coordinating a conspiracy - it signals to other protest groups that if someone starts suggesting violence, you should decline because they might be an agent. That could then prevent violence in cases where no undercover agents have infiltrated the group. I still think the tactic is unethical, but this is one way it could be effective.