Well, as an LB, you also want that speed to beat that RB or running QB to the corner....
Having speed isn't a bad thing, but 40 times, as you mention, don't allow for anticipation, ability to shed blocks, read plays, keep one's feet through trash, etc.
I mean what do you think is the 40 time last year for James Harrison with those bow legs? God love him (I know I do, he's one of my favorite players, both for his hatred of Goodell and how he plays the game), but there's no WAY he runs a sub-5.0 40. Just no way...
And yet... That monster can flat out ball. He's a Skeeery man. He's not as slow Jason Vorhees, but he's about as skeery and you can almost hear the "ki-ki-ki....hu-hu-hu" as he's tracking a ball carrier down. And as hard as he hits, I'm kinda surprised one of them hasn't wet themselves...
That's a long way around saying that for me, 40 times aren't the whole shebang.