These days I rarely use my IS at all (well once this year) on any of my IS lenses.
Whilst I fancy the 16-35 F4 very much I struggle to think of a use for the IS for my photography, I don't use it on my Canon 800 F5.6 L IS even when hand holding so on a 16-35?
Still if people want it then Canon would be silly not to include it!
I recently spent 3 weeks in Europe traveling all over. Inside catherdals and many places tripods weren't allowed or to much of a hassle with so many people. And let me tell you the IS on the 16-35 worked wonders for getting hand held shots in extremely low light. Without it Theres know way I would've gotten those shots hand held at the slow shutter speed I needed.
Coincidentally I was in France in September doing very similar things! I was using the 24-105 and 17-40, I can't quite afford the new 16-35 yet! When in France these sort of shots are compulsory as far as I am concerned. Admittedly I do use a 1DX so I can let the ISO rise quite a bit before it concerns me.
As stated, these days, I find very little use for IS, I am not saying it is not handy occasionally but I am getting more keepers on my long lenses without it and have little/no use for it on short lenses. As such I would speculate that the 16-35 F4 might have been an even better (and slightly cheaper) lens without IS and the extra element + bulk that it involves. Either way it seems to be a lovely lens and I will be investing in one as and when I can afford one!