Once in a year (or two) I get a chance to shoot Tennis, Moto GP, Drag Races & Speed boats (when they happen in this country) ... unless one is a professional who gets to sit/stand in a good spot, I think zooms are more convenient ... I once tried shooting a tennis match (Maria Sharapova vs Vera Zvonareva) with EF 300 F/4 lens but kept hunting for the players because I couldn't locate them with just one large focal length .. also tried with 100 f/2 but found it too short ... the 70-200 is a great lens for hobbyists like me who don't always get to sit in perfect spots ... although IS does not really matter when you are shooting at faster shutter speeds, it is reassuring when what you see in the view finder is stable (with the IS on) and your focus point is exactly where you intend it to be.
Obviously a prime would give better results (only if you know where/how far you will be seated from the action) but a zoom ensures you don't miss a shot, due to its versatility.
