I just realised this too during the week. I had always wondered why the kinematic roll centre isn't valid since it is merely the application of 4 bar linkage theory twice.
The reason why its not valid is because like Z mentioned, the tyres aren't pin jointed to the ground at the contact patch. Therefore, if you use the geometric RC as a motion centre about which you recalculate the body's roll intertia, its not just a simplification but IMO a massive error.