lol, they are RB's and LB's they usually have to keep up with wingers and outmuscle them, if they are too slow, the wingers will fly past and you will get cut open.
if they are faster than the winger, they will be able to catch them and push them off the ball or tackle them.
also if your CB gets beaten your next line of defense is usually either the keeper or your RB or LB, but they have to be fast enough to move from the wing to the centre of the pitch before their striker has scored. in order to do this they must be fast.
put it this way, ive played about 30 games this week and not lost one.
my RB is rated 77 overall in my first team, go figure, but he has 85 pace.