The problem is that defenders don't just line up next to you and let you outrun them sprinting in a straight line to get open. This will become more and more of an issue with the success of Seattle's large, physical DBs (who also happen to be defensive backs) in a copycat league. Now don't get me wrong - 40 times are a useful tool in the evaluation process, but should be far from the top thing we look for in a guy.
Guys like Boldin and Rice have shown that even a 40 time of greater than 4.7 won't stop a player from becoming an elite receiver. I would also argue that a lot of the speedsters that have had a lot of success in the NFL have done so because of other attributes. In other words, fast 40 receivers who have been successful in the NFL would still be successful (albeit less so) if they didn't have elite speed but kept their other skills, but would not be successful if they lost other skills and kept their speed
Other skills like quickness, agility, size, catch radius, high football IQ, low drop rate, route running, ability to pick up contested YAC (i.e. not in the times you just outrun a guy, but rather juke or give a stiffarm or fall forward for a 1st down, etc), ability to disguise routes, changing direction/cutting at high speed, durability, etc are all just as important or more so. Give me the 4.5 or 4.6 guy who does these other things well over a 4.3 guy who does these things OK or struggles in some areas
This is especially true for our 49ers, and we can look no further than the recent past to prove it.
The receivers we have had the most success with, by far, have been guys who are far from burners: Crabtree and Boldin. Vernon Davis is a burner, but is not a receiver and much of his success is predicated on the fact that he is a TE and doesn't have to line up outside, but rather inside against linebackers and safeties, where his speed difference is much more drastic. Guys like Ginn, Kyle Williams, and the infamous AJJ all had much better speed, but failed to make much of an impact in our passing game.
There reason for this post is that I keep reading that we need to add a deep threat in a receiver (which I agree with), but that gets interpreted to mean the faster the 40 time, the better the deep threat. This is definitely not the case, and has proven so many, many times. The biggest busts at receiver don't happen when teams take a guy with adequate speed but runs great routes and catches everything, but rather when they take a speed demon who they fall in love with and end up overlooking other glaring weaknesses. I just hope our 49ers take a long, hard look at a guy with a fast 40 but other glaring weaknesses like Cooks (undersized) or pass on a guy like Landry who ran slower, but brings a fearlessness over the middle, physicality, blocking, and route running that most of the guys ahead of him can't match