It can be relatively dark in the canopy of the woods when going birding, even on a sunny day. I'm not catching warblers in flight in this scenario, they are always in trees, bushes, etc. So in this (relatively common) scenario, getting my 400mm down to 1/50 of a second would be awesome for me. Even if I had the 100-400 with IS, I'm guessing that I would still welcome the IBIS.I was thinking in terms of the subjects. 400 mm primarily used for wildlife, birds and sports. I suppose for a large slow moving mammal at rest or a bird sitting in a tree you might be able to shoot at 1/50 of a second, but that's not generally my experience.
I would guess that it would be much less useful for action sports or BIF photos, but I don't have experience there.