At my organization, we maintain a number of different services that are used by a variety of clients. We need to bill clients for the services they use, so two questions come up fairly frequently:
1) Which users are using service X, and
2) How much is user Y using service X?
It would be very helpful if, in addition to the statistics on overall service usage available from ArcGIS Server, there were also a breakdown of which users were making requests. This functionality is also helpful when working with deprecated services, because it provides an easy way to see which users we need to contact about moving to new services.
The requests are generally already using the ArcGIS Server or Portal authentication, so logging (timestamp, service, user) and then getting statistics on that should be fairly straightforward.