The problem is that many of these other defensive appliances are rarely monitored. That is a sad but true statement in many cases. Furthermore, even when these appliances are monitored there is a strong possibility that the person who is reading the output does not have the requisite training, or knowledge, to understand the information they are seeing. In some cases, large networks receive hundreds of thousands of alerts every day.
The problem of an intrusion detection system going unmonitored, or misinterpreted is unfortunately an all too common one. Too many corporations invest in the technology yet do not invest in the human side of the equation to manage and monitor the equipment.