All they do is call the real police, the ones that we are already paying for with our property taxes.
It seems like a better solution would be to make the real police answerable to the will of the people. Perhaps these neighborhoods should focus on fixing their political processes rather than building a parallel police dept.
Last October my neighbor invited both our city councilwoman, and her opponent, to a neighborhood meeting at his house. About 80 people showed up. We grilled both of them on what they were going to do about recent burglaries. The result was the councilwoman was re-elected and we got a specific cop assigned to our area. He is "our cop" and works with our neighborhood watch group, personally follows up every crime report, and attends our periodic neighborhood BBQs to give pointers on crime prevention. If you pay taxes, the police work for you, and you (collectively) have the power to make them do their jobs.