For events, wouldn't the focal range of 24-70 mainly be utilized for taking still shots of groups of people posing for a shot with or without flash where IS would be more handy especially in low light/indoor locations? Honestly I would think the percentage of motion/candid/unposed shots at an event where IS would not be needed would be a pretty small in comparison to the overall batch of event shots?
People move all the time. Even breathing involves some motion. In a wide to normal unposed shot which is likely to contain multiple subjects in the same frame, the chance of someone moving enough to not only make the picture less sharp but also introduce visible motion artifact is that much higher.
Many photographers like to use a shutter speed of 1/60 or faster for this reason. There is little need for IS when using a standard zoom at this shutter speed or faster. Professional event photographers will usually use a flash if there is not much available light. If IS is needed
for most poorly lit events, why do so many event photographers choose lenses without it as their standard zooms ?
For longer shots, you not only have a better chance of locking in on a single stationary subject, a shutter speed of 1/60 or so is probably too slow unless you have IS or use some support.