In years past, treading between the numbers was a danger zone for receivers, who expected to be re-routed if they tried to run a crossing pattern or to take a potentially concussive hit once they caught the ball. With officials being more cognizant of penalizing helmet-to-helmet hits, players have backed off, making the middle of the field a far safer place to play.
For proof, look at the New England Patriots. Wes Welker, Gronkowski and Hernandez work the middle of the field like an Etch A Sketch. They exploit matchups, talent and rules arguably better than any team in the league — especially with a pinpoint passer in Tom Brady.