Using the ITracingService only works for plug-ins registered in SandBox.
Since this service is so much better/easier to use than e.g. logging to file on the server, this should be enabled also when using plug-ins not registered in isolation.
MSDN article describing where it should be available: "Tracing is supported for sandboxed (partial trust) and full trust registered custom code and during synchronous or asynchronous execution"
Debug a plug-In - Logging and tracing
Duplicate idea: https://crmideas.dynamics.com/ideas/dynamics-crm/ID0000828
Very useful to have this feature enable for non sandbox register plugins