Alot of the combine drills are really to show what you CAN'T do. If you have bad technique the drills will expose that, because of alot of stuff you can get away with in college that you just can't do in the NFL. Players that are amazing in college (Ryan Leaf, Akili Smith, The Boz, Andre Ware, Archie Griffin (He won the heisman twice) Kitjana Carter ect ect)
might not have the skill set that translates to the NFL, the combine can help decipher some of that and say, "Oh he can't throw from a 3 step drop, or Oh he can dominate guys worse than him, but he just doesn't have the motor to compete at this level"
As for the 40, if you were a consistant producer, and are doin well in the other drills, coaches will let a 40 slide, but if say Percy Harvin had come out and Run a 4.7 then he would have dropped severly, because u need to have the speed to compete at this level at that position, and anything less that a 4.4 maybe 4.5 will make coaches take a 2nd thought.