When it comes to arm strength, the issue is not about throwing 50+ yards. The test of a good QB's arm is different. Like most issues involving the passing game, it's best evaluated in the medium passing game.
The question is this: can a QB deliver an accurate ball to hit a tight window, say, 35 yards away, before the coverage closes and with a quick flick of the wrist.
Limited QBs must gather their feet and step into a throw, requiring a luxury that pass rushes often prevent. And, limited QBs float the ball into that window, giving DBs extra time to break on the ball.
A QB with a quality arm can glance left or right, see a window, get the ball off without ponderous footwork, and zip the ball in there before the window closes. This is exactly what is meant by the "throw in a phone book" quotation is talking about.
As is being said in this thread, one does not need a canon arm to do this. But, one needs a quality arm. Pop-gunners who have take a big step into the throw and still float the ball on a sideline throw are and always will be limited.
A good comparison in the thread is Montana. I'd add Kenny Stabler, who was known for "throwing darts." Those guys did not have the canons of Elway or Bradshaw. But they could make plays all over the field and under duress.
I always get frustrated when the discussion focuses obsessively on big bombs downfield. Quality arms are established in throws that may only advance the ball 12-25 yards but must travel on a line with a quick release off limited footwork.
