It seems like this common courtesy is not taught anywhere. Folks at work didn't know it. The cute blonde who just asked me for a cigarette (I don't smoke, btw) didn't know it. This crude picture below should explain things.
In the above diagram, my car is the blue box and the blue lines represent my field of view. The red box is (and always seems to be) a full sized truck with an extended cab, raised platform, tinted windows, side and ground effects, and a pissing Calvin somewhere; the red lines are his field of view. The yellow boxes are oncoming traffic and represent 200 points for a clean hit, 150 for; oops I spoke too much. The left section of the diagram represents what happens when dumb-ass in his Canyonero pulls up to see beyond me. I can't see through his vehicle the entire time he is there waiting there. The right section represents what happens when the same dumb-ass stays just behind me; he can still see vehicles coming from the left and doesn't block my view of traffic in either direction. But wait, there's more. By hanging back you get an extra five or six feet to accelerate, providing you with more opportunities to get to where you are going.