While I have trouble imagining it making that much performance
difference going from 4 verts to 3 for a whopping 2 polygons, or even
from 2 triangles to 1 for each poly, using only indices for the vertices
does remove a lot of code, and better yet, some memory and buffer
allocations... always a good thing.
That said, I guess freeing up a GPU thread for something else could make
a difference.