I fully support having an organized, armed, militia-type group of citizens that could step up to help in a true SHTF scenario.
As a ranking cop in a decent sized city (and a rather large total metropolitan area) I know for a fact that we will not be able to effectively safeguard our residents in a large, national (or perhaps even regional) event. Our officer to citizen ratio is close to 500:1. We get run ragged on just an average summer night. If there were mass uprisings/lootings/riots, etc., we would only be able to maintain a bit of order for a short while. Obviously, if it is a local or smaller regional situation, we can invoke mutual aid from other agencies and the National Guard, but that is only if they aren't tied up with their own messes.
So, with that being said, I think it would be great if there were a cadre of citizens prepared to help out. I don't think it will ever happen, though, on a large scale. I think there are too many legal, logistical, and command and control issues. I can't envision a city/county/state attorney signing off on it or the council/commissioners endorsing it. What happens when someone gets hurt while training to help the city? Who do they report to in an emergency? Each group led separately, with it's own command structure? There are too many details to work out.
My opinion is that it should be done on an unofficial level. Instead of seeing cops as the enemy, get to know them. Volunteer as auxiliary officers, attend the citizens police academy, join a CERT team, participate in neighborhood watch, etc. Offer to let your local LEOs use your shooting ranges. Once you get to know them, and they get to know you, let them know that you have a group of friends who train together and are prepared to help out in an earth shattering, EOTWAWKI, situation. That way, even if they don't call on you for help, if things are ever bad enough for you to mobilize, they'll know who you are and that you are on the right side of things. Clearly, this is better suited for a small town/rural county than a big city, but it's a start.
I think the biggest hurdle for militia groups is just being able to define which ones are the good guys and which ones are trouble makers. That's where networking with your local officials will come in handy. Also, having ex-military and stand up citizens will also help. I'd be more inclined to trust a group with professionals, ex-military, no criminal histories, etc. than I would than I would a group of drunken hillbillies (which I'm not knocking, I have a few in my family...)