At time of asking the question, the answer from @LarsSkovslund was correct. However with the stable version of Microsoft.Diagnostics.Tracing.EventSource, Microsoft changed this according to their blog post:
With the RTM release we’ve relaxed some of the event source validation rules in order to enable certain advanced usage scenarios.
The two changes in this area:
EventSource types may now implement interfaces to enable the use of event source types in advanced logging systems that use interfaces to define a common logging target.
The concept of a utility event source type (defined as an abstract class deriving from EventSource) is introduced to support sharing code across multiple event source types in a project (e.g. for optimized WriteEvent() overloads).
System.Diagnostics.Tracing.EventSource, provided with the .NET Framework, supports these scenarios as of .NET 4.6