a lot of quarterbacks throw downfield with a lot of pressure right in their face, sometimes fractions of a second before they get leveled!
the oncoming linemen/ blitzers are not always going to be running at the qb standing straight up, so while they might be infront of him, a qb that is standing up tall in the pocket would probably be able to see over the athletic pose of a lineman bombing in to get him.
even if his line is holding and every one is more upright the qb should have time to shift around in the pocket to find 'windows' to see down field (the qb at my school is listed at 6'1"... eveyone knows that means he is 5'9"... he is shorter than his line and plays almost on his tiptoes sometimes, and with one heck of an arm He is the second ranked passer all time in canadian university football
, I wouldn't worry about straight sight lines, a good qb can find them...if he couldnt the team would find one that could.)
I see the pressure acting more on the time that the qb can look, making him throw too soon when things havent developed yet.