Summary
IDS systems are useful, and most likely necessary in the same way that firewalls are. If you are connected to the net, and have data that is valuable, this is a way of protecting it.
These systems are still emerging, after many years, and still have a long way to go. The problems they are trying to solve are non-trivial.
Makes for interesting research, and they definitely don’t harm an environment, as long as the operators do not assume that they are completely protected, and become lazy with security measures.