Currently FloatPoint contains arithmetic operations like addition and subtraction that FloatPoint3D lacks. Additionally, FloatPoint3D is missing useful operations like cross products that are needed for various geometric algorithms.
Created attachment 66086 [details]
From the ChangeLog:
Added set, move, and scale operations similar to FloatPoint's, addition and subtraction operators, length, lengthSquared and cross product. These changes have been tested with new code to be added later.
Committed r66533: <http://trac.webkit.org/changeset/66533>