Network information with OneAgents are for the system where OneAgent is installed. There are times where the technology (programming platform) or access to install an agent is not available.
DCRUM (NAM) provides system information from the traffic acquired from network spans or taps. No agents are needed. NAM can report details (operation requests/responses) on specific servers in the traffic or auto-discover servers used in the traffic thereby profiling network use. NAM does not provide the level of application detail as OneAgent does, but if OneAgent cannot be installed - some information is better than no information.
Perhaps some of these reasons are why you are using DCRUM (NAM) instead of Dynatrace and OneAgents.
Also keep in mind that DCRUM/NAM fits better in some technologies like Citrix, SAP modules. Also you can monitor your custom protocols using the Universal Decoder.
And when talking about cost it is also important to value both options.