Linux has native SNMP monitoring for trivial integration to existing monitoring systems.
So does Windows (although it also has its own)
All OES services are an invisible black box, unless I individually manage the local Nagios on each server or start using Nagios and/or Ganglion depending on the product. These tools are free but increase the effective cost of using OES or Filr as I have to manage individual servers or install new monitoring tools just for Novell products.
Adding SNMP means I can use the enterprise class monitoring tools I already have.
I can already use SNMP to monitor NetIQ products like eDir, Access Mgr etc