We have just recently got familiar with External Simplified Representations in order to keep our upper assemblies clean and free from orientations, cross sections and simplified representations that are mainly created for the drawings. External Simplified Representations offer a solution to organize each rep, orientation, ... specifically for each drawing. And that's a good thing.
In our assemblies, especially lower level sub assemblies, we have created user defined simplified representations where smaller parts are excluded, general parts are in geometry representation and even some parts are in a specific representation excluding complex features. That saves us quite some time when defining a simplified representation of higher level assemblies, where you can re-use the simplified representations available on lower level assemblies.
We noticed however that when defining an External Simplified Representation, you cannot reuse the lwoer level user defined simplified representations. Given the theory behing ESR's where you don't open the complete assembly but have a notion about the relations etc..., I can imagine it 's far more complex to use user defined reps compared with the default geometry, graphics, master reps, but still I cannot see a reason why is shouldn't be possible to reuse the existing simplified reps.
It is particularly inefficient when there are assemblies that are present numerous times in the modeltree.
One work-around we've been introducing is loading the complete top-assembly in graphics or geometry rep in the ESR, and then in turn create a simplified representation of that particular ESR. In the modeltree for example, we can use predefined simplified reps of sub assemblies and then save the newly created simplified rep into the ESR. This comes pretty close to reusing predefined reps in External Simplified reps, but is quite complex and doesn't offer the full benefits of ESR's. Furthermore, I don't want to be the person in charge of training new employees and explaining "you have to create a simplified representation in an external simplified representation in order to reuse predefined simplified representations".