Without knowing anything about OPNET, I see the following advantages for PI MCN Health from the perspective of a customer owning a PI System:
The PI MCN Health package is already included within the current PI System license type (PI Server). PI MCN Health interfaces are native PI Interfaces. There exists a PI System Management (SMT) plug-in for configuration. The PI ProcessBook integration allows automatic display creation.
2. Health History
Per default PI Points serviced by MCN Health interfaces are configured for archiving. This helps to see how a particular issue may have developed over time.
3. Further data treatment
Because of the integration into the PI System you can take advantage of using calculation engines (PI Performance Equations, PI ACE, PI Totalizer), describe your infrastructure within AF, create Event Frames, Notifications, use PI Alarm for further data treatment.
Besides ProcessBook you can use other clients like PI DataLink, PI Coresight, PI WebParts ...
There are no additional licensing costs except those for additional PI Points (if needed).