Why when a lense come with a focus ring, does the camera it's designed for not come with a split prism? How else do you judge focus? That little green light in my viewfinder with "focus" underneath it? Great it's in "focus"...WHAT is in focus, I can't see! </rant>
If you RT...M, you'll discover that if you manually select a focus point, say the center one, that the "in focus" indicator will be based on the selected focus point. And the higher-end cameras (5D, and all of the 1-series bodies) do have an available split-prism focus screen. The reason it's not the default is because it can negatively affect metering accuracy when spot metering with the center metering sensor under certain conditions. But that's not an issue when using evaluative or center-weighted metering, or spot metering with a non-center metering sensor (something you can do on the 1-series bodies). The 1-series bodies also have a larger, brighter viewfinder more suited to manual focusing even without a split-prism focus screen.