Adding in runs as completion skews the numbers in another way - there is no way to have an uncomplete run. If you run, you get a completion, if you throw, you have a chance one way or the other. If you were to make an accurate "QB rating," you would need to scale it such that a "run completion" did not equal the same thing as a "pass completion."
I assume you are quoting Ethanga62890 by the way. I was just in the process of writing this same post to the Falcons board.