Bump! Bump! I'm talking to myself, again.
Having used the product now for a few months and looking at a variety of things, I believe that Linked XIN with Native Styles is a better approach than 100% element templates. One key thing to understand, is how do the linked XIN Features get used. And which items can only exist as Element Templates.
As near as I can tell, once the linked XIN DGNLIB is created, you can rearrange the features into logical groups and it will still connect to the native style for symbologies.
Then, if you need to add features, you could link the revised XIN to a scratch DGNLIB and once the new styles are created, copy them from the scratch DGNLIB into the linked XIN DGNLIB. What I am not sure is that once you copy the feature to its new home, will it "see" the new Native Style in the original Linked XIN or only the scratch DGNLIB XIN and will it be easy to switch the Native Style to the other DGNLIB?