there are lots of times when you could find yourself filming handheld. when i went to south america i wasn't able to bring a glidecam or tripod so i had to film handheld and the IS helped a shit ton.
plus if you're using a telephoto, even on a tripod, the IS will help eliminate small shakes from pulling focus or anything else that could make the camera move a tiny bit.
that being said, i don't think IS is essential and I have bought non-IS lenses in the past to save money, but if you have the cash then why not just get the IS