Windows Event source msut be registered on each and every WFE server in the SharePoint Farm. So if you have a farm envrionment with 2 or 2+ WFE servers, you must not just create the evernt source inside a FeartureInstalled/FeatureActivated event handerler
since those evernt only get fired once in one of the WFE servers. To make sure event source is crated on all WFE servers, you can run a timer job whitch calls
DiagnosticsAreaEventSource.EnsureConfiguredAreasRegistered. You can also run a console app in each and every WFE server to create event source.
Areas and category are stored in the SharePoint database, so they only need to be crated once and you can create them in a FeatrueInstalled event handler.
One use of defining Area/Categoy is for the enterprise to used predifined logging tree structure to enoforce standard practices. While we also keep it flexiable for the reason that you just mentioned.
As far as area/category strings, you can use many ways to make them flexible. For example, in the Logging Hands on lab
SharePoint Guidance 2010 Hands On Lab, it defines a string constant for area/category so you don't copy the acutaul strings around. I am sure you have ways to make it more flexible.