Since the flash meters through the lens, it is irrelevent for the camera to know about the flash modifiers, and the newer EOS flashes do take in consideration the distance from the subject, and it weighs heavily on the focus point the shooter chose. In addition the flash does not care if the it is bounced off the ceiling or wall, again because it meters TTL. All this conspires to create a flashing system that is right on perfect most of the time. The flash works closely with the chosen focus point and this conspiracy results in excellent exposure metering. In addition, the "input of all the variables" is mostly automatic, so the shot will not be gone by the time the camera workflow is done.
There are thousands of EOS pro shooters that rely heavily on E-TTL for millions of wedding and other types of shots with awesome results.
I love that one can slave many other EOS flashes and E-TTL supports this automatically, and E-TTL allows the photographer to focus his mind mostly on setting up the shot, composing it...manual flashing is old school and no longer needed save for the very specialized and rare shots. Little need for hand held light meters when using TTL bodies such as the Nikon's and the Canons (later film SLR models only).