Volunteer citizens' militia could serve as a useful purpose, as long as it is regulated. Much like the debates on weapons/gun control, the person you select to protect your community or neighborhood should be subject to qualifications. The police cannot be everywhere. Giving the power to defend a community would be a good idea, given their actions are not above the law.
In many communities we already have some equivalent of volunteer groups, people who are volunteer fire fighters and the like. In my community we have a volunteer police organization that sort of cruises the community and watches for trouble. I'm assuming a volunteer militia would involve people that are armed and capable of dealing with far more intense situations. I just read another contributor mention the National Guard that comes in to restore order in an emergency. I do think communities would benefit from having a similar force on a more local level, but it would have to have some controls in place to see that it did not overstep its bounds. It sounds like one of those ideas that have its benefits and its setbacks, but something that could be considered.
There are times a volunteer citizens' militia can serve a useful purpose, but the one that gets the most discussion isn't one. Gun nuts that think they can pick up their pistols and hold off the United States military are legitimately insane, but there are smaller, more localized cases (most of which being natural disasters) where having a trusted group of quick responders could potentially be beneficial.
A volunteer citizens militia with an establish chain of command under the governor of the state would serve a purpose we currently call it the national guard. Assuming we are asking about smaller more local militias all that would do is increase the reaction time in the local community so yes any opportunity for neighbor to help neighbor is a good thing.
We already have a professional military, both in the full time sense and in terms of reserves. These are people who are well-trained and compensated and also screened out and disciplined if they behave poorly. A local volunteer militia would attract all kinds of people whose goals and desires may not be cautious protection.