If you have only cucm cluster then don't really need of it but you must/should have configured cucm with DNS if you require imp, expressway, jabber etc or planning in future.
We are using 01 publisher 03 subscriber (across the WAN) and then CUC, IM & CUPS, Expressway and Jabber. But CUCM is not configured with DNS and domain and everything is working fine.
Deployment can behave abnormally, as per cucm and imp installation guide.." If your deployment uses DNS—CUCM and IM and Presence should use the same DNS server. If you use different DNS servers between IM and Presence and CUCM, it is likely to cause abnormal system behavior."
It is not mandatory to configure a domain and DNS server for your CUCM cluster. Refer to the following link from SRND which mentions about deploying CUCM without DNS:
this is quite a big question although the SRND indicates we do not require DNS I have found that we can encounter some abnormal issues, I personally would deploy DNS but make sure you have HA in your DNS environment.
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