ellipsoid - "Horizontal" and vertical datums terminology

I have heard people referring to as a horizontal datum to models such as WGS84 and as a vertical datum to geoids. The former is primarily used for determining latitude and longitude but also the height (and thus X, Y Z can be calculated) above the ellipsoid while the latter is used for determining height above an equal-potential surface only.

I wonder why one would call "horizontal" a full 3D datum. The fact that you have a pure vertical datum (i.e. geoid) does not mean you have to call "horizontal" the remaining one.

Here are two articles where you can find the term:

http://geography.about.com/od/geographyintern/a/datums.htm http://www.esri.com/news/arcuser/0703/geoid1of3.html