Subj, it reduces number of state changes and cache flushes. Because same material uses less vertex buffers.
VertexBufferManager now has a coupling about VB types, it's not really good. But a more proper way requires a much more rewriting. The current solution can be improved in future without rewriting, one might use some kind of custom allocators.
The change might add few MB overhead (more precise `Group::COUNT * std::min(MAX_VB_SIZE, MAX_VERTEX_SIZE * 65536)`), but it's ok since we already use hundreds of VRAM for textures.