Why would you need to update dietpi if you only run Roonbridge? I have PIs with very old Linux distros and RoonBridge just updates itself and works like a charm.
I am not a linux expert and would not know why I would have to do what dietpi itself tells me to do when an update is available. Might be a bugfix?
In my Linux noob view there is a distinct advantage if Roon takes care of keeping everything up to date like they do with ROCK.
I understand your concern. My view is: never update dietPi unless the system stops working (very unlikely with only RoonBridge). If by any chance it stops working, just reinstall the latest version