Network Intrusion Detection Systems (NIDS) offer several advantages. They provide insight into network traffic, allowing for the monitoring of data as it traverses the network, which aids in identifying unusual patterns or suspicious activities (B). They also help in detecting network operations issues, as they can identify performance-related problems and potential vulnerabilities (D). Finally, NIDS systems can be relatively inexpensive to manage compared to other extensive security measures, making them a cost-effective solution for network security (E). Therefore, the correct answers are B, D, and E.