CesiumGlobeAnchor position is currently expressed as lat+lon+height (WGS84) or ECEF xyz.
For most actual world use, the position of a 3D point is most probably given as a lat+lon+altitude where lat+lon is relative to WGS84 but altitude is relative to MSL (EGM2008).
Most DEM (geotiff or DTED or asc) are expressed this way, Google Earth is this way, etc.
Using WGS84 for altitude is almost never what we want as user, as can be seen by a long list of report in this forum or elsewhere on the net.
It would be most certainly very useful to switch to a MSL altitude, or at the very least to give it as an option in CesiumGeoReference.