Well the Guru of modern day Strength and Conditioning Programs, and the father of Husker Power, Boyd Epley would disagree with you.
Q: What is another problem you see in regard to strength and conditioning?
Epley: “We've focused on 40-yard dash all these years. Actually, 10-yard dash is much more important in identifying talent. Coaches are just hung up on what they've always done. If you don't beat an athlete in the first few steps, it doesn't matter what your 40 time is."
Omaha World Herald - Q & A with Boyd Epley: 'Tailor athletes' training'
I agree 100% with Boyd about the importance of those 1st 10 yards.
However, is being great at a ten yard dash any good if you can't sustain that speed for 30-50 yards?
I dunno, it seems to me that in a situation where a RB is racing down the sideline towards the endzone, a LB or DB's 40 time would certainly come into play because they're trying to catch him from behind.
Like I said, the 40 has been over-hyped...but it ain't meaningless either.