It's about making a political statement as much as it is "helping" patrol the border. The political statement needs to be made. As far as assisting in border patrol, what is stated by this article may actually be the case.
just the rumor of these groups showing up is a deterrent, since these groups routinely make national news in Mexico, and there is always a paranoid tone to the reports that "vigilante groups are hunting immigrants" . It must be in the Mexican press' style books, but the word "hunting" is always used in reference to the vigilante groups' activities. In the absence of action, which we can't get from Washington, at least there is some propaganda generated from all this. And the Mexicans generate it, which is even better.