One thing that is great about BBs, and more specifically, BB speed, is that when you break contain up the middle, nobody has an angle on you. In fact, you have the angle on most of them in pursuit. That is part of what made Dwyer so effectively fast. 4.6 is enough speed, if you are breaking free straight up field in the middle of a defense. Not so much, if you are taking a pitch up the sideline, where every defender has an angle of some sort. Dwyer was deceptively fast AND he was often racing against players without an angle.
I love BB speed and think it is a great help. However, the MOST IMPORTANT quality is being really tough to put on the ground. Give me a pile pusher first, then, if he has speed, great.