The very presence of armed protestors is inherently disruptive. Police, the KKK, and other enforcers of malice typically don't engage non-violent protestors when the risk of being harmed themselves is a possibility. Armed protesters are guardians, who simply promise that violence will be met in kind.

Unfortunately, there are many "moderate" members among peaceful protesters who can only think in binary: There is either peace or violence, and being armed defaults to full-on violence in their eyes. Personally, I am of the opinion that such a position is worse than useless when demanding for peaceful reform.

If you cannot retaliate against the opposition if they decide to use force, they have no incentive to negotiate. Those who enjoy power only respects power. Purely peaceful protest movements that drive out those willing to bear arms from the cause, will result in two things:

1: Less unity and power for the movement.

2: Reduced ability to lay out rules of engagement for armed members of the movement, because the armed and unarmed wings of the reform movement don't interact.

It is very important for peaceful and defiant wings of reform to cooperate, not to be isolated. Without both wings, the movement cannot fly.