The right of individuals to organize is, thankfully, protected by the first amendment. Whatever their motives, the Minutemen have a right to stand at our borders. Provided that they do not physically assault anyone, or trespass, or devalue anyone's property in the process, then they can't be shut down just because they are inconvenient. (And they are inconvenient -- bad as coyotes for setting off ground sensors and other alarms.)
However, an incompletely closed border can cause unexpected consequences, often merely routing traffic to other unobserved areas. And a firmly closed one would noticeably reduce US productivity (sorry for linked page's ugliness, but unless you have a WSJ subscription google cache is the best i could do.)
The current system protects neither our borders, nor our economy, nor the many thousands of workers passing to and fro. Kennedy and McCain are backing a bill for immigration reform that i can't find the text for. (If anyone gets a look at the bill itself, post a link?) Folks seem split on it, claiming alternately that it piggybacks Pres. Bush's ideas or that it is merely more lipstick on the pig. Whatever side of the fence you are on, it seems pretty obvious we need a better solution than Jim on his day off.