Well, I have never seen a WR run a 5.0 40, so the 40 time shows some threshold of a player's ability. If Chris Johnson runs a 4.6 instead of a 4.24, he doesn't get drafted and certainly doesn't run for 2000 yards in an NFL season. No, a 40 times doesn't matter by itself, but it shows a player's potential capabilities and how that can translate on to the football field. So who gives a crap about 40 times? Every NFL coach, scout, GM, owners, people who evaluate players for a living - exactly what we are doing here, evaluating a player.
Anquan Boldin ran a 4.80 40 yard dash......
Jerry Rice ran a 4.71 40 yard dash.... he turned out to be pretty decent.
Source: http://www.jetnation.com/2013/04/03/are-nfl-prospects-40-yard-dash-times-overvalued/