The standard lens requirement for shooting events/weddings seem to be a combination of 24-70/28 and 70-200/2.8, I read both are nailed to a pro's camera 90% of the time (though I have problems doing the maths :-))I disagree with your basic presmise that 24-70 and 70-200 are "nailed" to wedding photographer's SLR's...and are "must-have" lenses. This is simply not accurate.
Well, I actually I do agree with you and have to admit that this premise was an ironic part, 90% + 90% = 180% :-)
Another is an old timer who can pick his L-lenses from his oversized bag, but what sticks out in my mind is not his lens collection...
That's what a photog also said to me, but on the other hand the "I can shoot a wedding with my iPhone" photog type is a bit suspicious to me, too - I really can't tell what's reality and what's showing off in either "top gear" or "low gear" way.
I thought that 2.8 was important for using the high precision cross type auto focus sensors.... Better and more accurate focus at 2.8, then the pics append at the aperture of choice... Correct me if I am wrong, but I thought that this was how the autofocus operates....
Afaik: Yes and no. The 5d3/1dx-type af is more precise with the newest lenses (*not* necessarily the ones with the widest apertures, see lens groups in the manual), i.e. it can use all focus points, the doublecross sensor points don't degrade & the enhanced motor precision can be used (see lensrentals test on the 6d af which cannot). So that's +1 for the newest Canon f2.8 zooms.
On the other hand the camera *has* to focus more precisely when using a f2.8+ lens, and that can cost speed as a tradeoff for precision - so focusing with a "dumb" f4 lens might be faster since a little focus miss doesn't show.
And the 6d/5d2 is another matter, it has *no* cross-type af point @f2.8, but falls back to the less precise cross when the f2.8 line fails to focus - so -1 for f2.8 lenses since this af system is "anti-tuned" for it, there's a reason the Canon sample shots are made with the 17-40/4 and the kit lenses are 24-105/4 and the new 24-70/4.
Turning this around, what are the disadvantages of f/2.8 zooms ?
Um, price, bulk (scare-factor, pack-size) and weight come to mind :-p that's why I have a 70-300L and not a 70-200/2.8.