You are right in that the NFL puts a ton of weight on the combine and how you do or don't do in it. But it still is a bit funny that a lineman might be okay if he ran a 40-yard dash in 4.6... but not if he did it in 4.8. That probably represents a 2 foot difference. Especially funny when you consider that he will never run an open 40 in a game.
But, unfortunately, that a tool they use to evaluate.