I am informed by the behavior of the Blender exporter for BabylonJS. If you assign a skeleton to a mesh, it no longer writes an optimized list of verts/normals/UV/etc. It writes them all. Now I do not really know why this required, but lets assume it (I will challenge this through experimentation @ another time).
This can create a choke point. As your mesh gets larger, if you have a skeleton you could reach the 65k limit of indices (likely causing problems for mobile), where as if they were optimized they would not. If you are not optimizing, would not requiring indices completely solve the problem?
That is if it works. There is much checking if indices are defined in JS, but will the other buffers work without the index?