besides the multi texturing, acc also stores surface normals (which ac does not). Maybe, osg could calculate them at runtime (i think, most of the normals are auto-generated by sd2-accc), but this would eliminate the possibility to use "faked" (or "tuned") normals to pretend a detail or shape that would otherwise need much more polys (no idea in how far this actually used for SD cars).
Also, consider that SD is in a kind of transition phase- i.e. osg support is still in development and and does not include all the features that were implemented for plib/ssg. For me, ssg still offers a better gaming experience (though osg's advantages are already visible). So- why not keep them (and run sd2-accc for new content) as long as ssg is still used by some?