I'm beginning to familiarize myself with Geometric Networks in ArcGIS Desktop, and according to About Verify Network tools and commands, multi-part features are not allowed, but they do not explain why. What is it about multipart features that breaks a geometric network?
Imagine a geometric network where multipart lines were supported. When tracing such a line the trace might get to a gap between the parts and then much more complexity to the model is introduced.
For example, you could make it support jumping the gap but the direction on the other side of the gap may be reversed.
I would say that the benefits of supporting multi parts in geometric networks would be greatly outweighed by the software development and user training involved.