I was taking a look at zoom lenses after using my friend's 100-400 the other weekend, and noticed that many of them don't have IS. I know the 70-200 2.8 has an IS version (at a significant cost) but a lot of the others don't have an IS version.

I would have thought that IS would be even more important in a lens with a longer focal length.

Is there a reason for not having IS in these lenses? Do you find that it makes shooting difficult without it?

Thanks in advance