Dear Scott,
I repeat my answer that I just provided to your later thread Re: API bug in Document.Regenerate:
http://forums.autodesk.com/t5/revit-api/api-bug-in-document-regenerate/td-p/5911198
I do not believe this is a bug.
This is simply a proof that you cannot store a PlanarFace.
How do you want to achieve that?
It is not possible.
Revit geometry is read-only, and represents a view of the parametric BIM, a momentary snapshot.
As soon as you add something to the wall, e.g. the family instances you mention, the wall is affected.
Regenerate creates a new snapshot of the current situation, and the planar face that you previously "stored" (I wish I had triple double quotes here) is in blissful oblivion.
I hope this clarifies.
Cheers,
Jeremy
Dear Scott,
Thank you for your understanding and hope that the further testing will clarify more.
Meanwhile, I published the currrent state of affairs on The Building Coder:
http://thebuildingcoder.typepad.com/blog/2015/11/c
Cheers,
Jeremy