The question as to why use an f/2.8 zoom rather than a prime is a qhole other question (the OP has repeated this point, so I'm avoiding repeating the argument for a fast lens).
Many people prefer to use primes as much as possible -- in fact the 85mm f/1.8 lens is a favored sports lens for its wide aperture and fast AF. But, sports and other venuse where you are limited in how close you can get to the action will not always lend themselves well to a fixed focal length. This is why you see sports shooters with multiple bodies, covering different ranges. Sometimes they will have a wider prime and a telephoto prime, but you also see the white zooms.
With medium distances, I don't think there is a good substitute for the 70-200 f/2.8 lens. With or without IS, it is just a great lens for action that can vary in distance within its range. It is a high performance lens for that type of work, and it takes a teleconverter nicely as well. For some stuff it's the only lens I use, but each scenario has different requirements.
All that being said, I enjoy, for instance, the 24-105 as a walk-around lens for the lightness and compactness. Plus, in everyday shooting, the IS really does help. So, there are plenty of times when the Big Guns go unused.