The only working Nvidia system I have at the moment is an antiquated, cheap little Ion netbook with a weak N270 Atom. So let see how it fares in the debate whether it is preferrable to use the image backend and push images to the server rather than send geometry via XRender.
The white baseline in the centre is the performance of the image backend on the N270. Above that line and the driver is faster and more responsive, below it consumes more CPU, more power and lags more than if we were just to render it ourselves.
What we can see is that as a result of Nvidia investing engineering time in their driver, on the whole, it performs better than we could by just using the CPU. Better performance means more responsive user interfaces that sip less power, meaning happier users for longer.