Here's a trick you can do to draw all your 2-triangle-quads in one draw call; pass 4 identical vertices for each quad, and 4 other attributes representing each corner of the quad. (-1, -1), (1, -1), etc. Then displace the vertex according to the corner attribute in the vertex shader. There's more data to pass, but I assume you're not passing the data every frame. If that's the case, don't. If you're moving the quads, just pass the position.

This is false. If you're drawing a quad, you pass 4 vertices. If you draw 2 triangles forming a quad, you also pass 4 vertices (using a triangle strip and index buffer). The index buffer is not updated every frame, just once.

That's a really good reason to have native OpenGL ES drivers IMHO. But why would you create an app on windows, and release it on any platform except windows?

There will be a windows driver soon, I assume. I also assume NVIDIA and AMD will provide drivers for windows. Asking what specifically you would use OpenGL ES 3.0 on windows for is like asking specifically what you would use OpenGL on windows for. It's for portable 3D graphics. OpenGL ES 3.0 looks like it will be the most portable version yet.

Portability. You could write a game engine that would run pretty much everywhere.

Just curious, how would quads be more efficient than say, triangle strips for drawing quads?