edit - and a faster lens is a godsend for action shots - if you notice my recent series they are all f2 with my canon or 2.8 with my sigma - that made for shutter speeds in the thousandths and completely motion-blur free action shots and it was overcast! ( i was using ISO 400 - which is still very acceptable in terms of noise)

Inshaala nailed it on the head.

You'll see sports photographers on the sidelines at football games with masive lenses with wide apertures because when all the way open they will have faster shutterspeeds to stop any and all action. And when the sun gets lower they'll need that fast shutter for shooting subjects in the shade. THEN when the sun goes down and they have to shoot in the supplied artificial light... You get the idea. It is for speed and speed alone.

The same thing is reccommended for those who shoot events like weddings to minimize motion blur in low-light situations - but with the advent of IS, OS, or VR systems (as well as faster ISO speeds - keep in mind this advice has been consistant from the 70's when a blazing fast ISO was 160!) this is less needed.

Any differences between 2.8 lenses and 4/5.6 lenses will be pretty much unnoticeable when care is taken for the quality of the shot - shoot the shot well and no one will know what was used.

Hope that helps-
-Lew ;-)