A bigger reason to do this is that on some (shoddy) hardware, the user sees a tear line along the diagonal of the triangles.
It's as if sometimes one triangle was rendered before the vsync, while the other was rendered after it.
It's as if sometimes one triangle was rendered before the vsync, while the other was rendered after it.
eg. Your shaders operate on float16 precision (common in the D3D9 days) on a screen with high enough resolution. The boundary pixels at the diagonal now can't fit into the precision of a float16 cleanly.
But the reality is that floating point numbers 'just work' for a huge number of things, and the space and compute inefficiency is small enough to not worry about.