Look at it from a galactic scale and you will note that we are not anywhere near the action when it comes to the resources this galaxy has to offer. If you are an advanced intelligence capable of interstellar travel and you need more resources then you are going to head for the closest stars and from those you would choose the one that put you on an optimal path toward the most dense parts of the galaxy. So what about Sol, where is our sun, off on the edge of the galaxy on an arm and near the edge of the arm so even if the consumptive nomads started their journey further out along this arm that we are on they would be travelling along the arms core, not near us. So how many likely system are so close to us that this logic does not matter, bugger all and then our signal may actually give the others a hint that they should choose the nearest other star to plunder as it is a lower risk proposition. In other words even if the "other" is a huge big heartless star eating robot the chances of it existing are slim and the chances of it finding it logical to come our way are even slimmer. Do the math, we are galactic hill billies and need not fear the traffic on the main roads.

Lamarck's Signature : How Retrogenes Are Changing Darwin's Natural Selection Paradigm Published Dec 1 1999 http://www.amazon.com/Lamarcks-Signature-Retrogenes-Changing-Selection/dp/0738201715