i’d like to see that i can define several UV, color and normal values per vertex with one glDrawElements call. so, if the vertex array contains n vertexes and the indexlist has m entries, the texture coord array, color array and so on can have m entries, too if it was enabled by e.g. glEnableClientState(GL_INDEX_TEXTURE_COORD_ARRAY). so vertexdata will not be transformed twice or more in well coded drivers and the app doesn’t have the overhead of innumerably function calls.
There was a pretty long discussion about a feature such as this a while ago.
Basically, it would be extremely difficult to implement this properly in a driver, and there would not be that much performance improvement.