I think that the title says it all. Why is this important?
Many of you are using linking on large multi-discipline projects. You will link all of your models into one 'documentation' model. If you cannot dimension to 'center of core' of a wall through a linked file you are missing a key documentation feature in the workflow.
You will have to draw and lock ref planes at the center of core in every wall, in every link. Then you will be able to dimension to the ref plane. ugh.
Autodesk Development is aware of this and plan to fix it. But to add some urgency, please contact Autodesk through your Subscription Center account or your local reseller and log this. If you have a better workaround please feel free to comment...