If you limit yourself to available light, a very dying breed nowadays, and you anticipate very low light levels where iso 5000 with a 5D MkIII won't cut it (macguyver's example was only at 2500 and f2. which is basically three stops over the best we used with film, then the f2.8 is your choice.Some good points, private, though I still think there's something unique (and better) about available light for event shooting, though I'd probably use lights for a wedding to avoid risking things. The new lens is definitely an improvement over the 2.8 II, but it doesn't make the 2.8 II a lousy lens and if f/2.8 were more important than sharper corners and lower CA, I'd go for the 2.8 II. For people who rarely shoot events, the f/4 IS is cheaper and better for everything else, and as you say, you can always add light
But those situations are few and far between for most of us. The truth is there are way more options open for us now with wireless flashes and superb high iso performance than there ever was with fast film, if I was buying new today I'd do what macguyver just did and sell his 16-35 f2.8 and get the f4 IS. Sure there might be situations where that one stop won't work, but we now have options, if you get enough function work where f4 limits you get a 24 f1.4 or a 600-EX-RT and ST-E3-RT to augment it.
I am a poor photographer, am still running the 16-35 f2.8 MkI, because we needed that with 800iso film!
I recently shot a wedding with a very dim reception, mood lighting was the term I think, dungeon would be more accurate, plus it was small with nowhere to put stands. I gelled a couple of 600-EX-RT's and put them on the tables and bounced off the ceiling, I also had one on camera, this made the weak DJ lights look much more effective and gave me enough fake ambient to work with my 1Ds MkIII's with their 1600iso limit (but I never go over 800 with them either).
I agree some shooters will always need more speed, but I'd advise any natural light specialists to go for 1.2-1.8 primes over 2.8-4 zooms anyway. I just get the feeling that there is a touch too much generalisation here from some people sometimes, we used to shoot 2.8 800iso images all the time, now many can easily shoot 5,000iso and that more than makes up for the one stop loss in lens speed for the same shutter speed value.
I really like your function examples, and you make the 2.8 point very well, but you could, if you had needed to, gone up one stop of iso, 2,000 to 4,000, and/or had a remote 600 popped into the canopy that would have blended well but not been obvious.