Hi Pierre,
I have a few remarks, since I am from realtime 3D world (and not really GIS), and I did not see in previous comments much about this.
I'll add to Bruno's answer nothing about gaming, because I'm not good at that. But just to show a small sample of a geometry created with GGL using transformations and 3D, in the GIS context, you can look at http://trac.osgeo.org/ggl/wiki/WhoUsesGGL About this:
# conversions of position/rotation/scale/normal ### for the following coordinates systems : world / view / parent / local
We have this in GIS as well. Within GGL you can present points in different coordinate systems. Even if they are both cartesian, then can be marked as different (within GIS this is e.g. the case for EPSG). The generic "transform" function can then transform (e.g. scale, rotate, translate) between coordinate systems, using matrix calculations. Regards, Barend