There is no predictability when using the Steering block. We don't want to teach our students always to find values with trial and error. We want to be able to teach them the principle that lies behind a certain behavior of the robot. With Steering block, we can't do so, and that is why we use it only for moving straight.
This is a companion discussion topic for the original entry at http://www.fllcasts.com/episodes/475-ev3-phi-teacher-note-when-to-use-tank-large-and-steering-block