Well, it appears that 2012 is the only version which holds the point's elevation when grip editing. Matt Kolberg tested versions 2009-2013 and reported these findings. I then tested in 2010 and 2011 myself and also found this to be the case. Sinc and Steve Boon also chimed in with similar findings.
It would be nice if there was a setting that we could use to enable/disable this, as I can see where it would be handy for either method to be used.
Jeff, would it be possible (in the mean time, waiting for Autodesk to formally fix this), to use an Event monitor for when a Point3d is Grip Edited, to store the Z prior to Grip Edit, and replace Z after Grip Edit?
It should also be simple enough to add a registry key, and CommandMethod to allow the user to toggle this setting, no?
... Still very new to C#, but conceptually this seems possible (to me).
Perhaps you can clarify.