i primarily shoot weddings (on the 5D2 and 5DC) and the reason i went with the 2.8 zooms was primarily that i found slower glass hunted for focus in low light far too often. i am still on version 1 for both the 24-70 and 70-200. the newer lenses coupled with the 5D3 might change the circumstances but i still wouldn't ever favor an F4 glass over an F2.8 or faster glass.
i rarely ever (probably never) shoot the 24-70mm at 2.8 because when i am using that lens i am typically trying to get more than one individual in the shot. 2.8 is too shallow a depth of field to get more than a couple people in focus so i am usually at an F5.6 or F4 (if i am desperate for light) on that lens. where i have seen the 24-105 hunt for focus at times in low light on the 5D series cameras, the 24-70 never hunts for focus for me. that is why i got it.
on the other hand, i will use the 70-200 at an f2.8 because with that focal length range i am typically trying to isolate a single individual. i prefer to use it an F4 because the results tend to be a bit sharper but i will push it to a 2.8 without much concern sometimes.
both those lenses are the foundation for the days shooting at a wedding for me....but, i would hesitate to call them my bread and butter. this past year i have added a 15mm fisheye, 35mm 1.4L, and 85mm 1.8 to the lenses that i bring on weddings. the fisheye i got just to get the occasional wacky overall shot (though i have found that lens to be extremely useful to me in my architectural shots...who knew!).
the 35mm F1.4 has really become my star performer for pre ceremony shots and couple shots. at F2.0 i find i can shoot a couple, keep them in the depth of field, and maintain really nice fall off to the background that i just dont get with the 24-70. a colleague of mine that i shoot with alot has quipped on several occasions that the 24-70mm is by far the most boring lens in her bag...and i tend to agree with her. but it covers alot of ground so for me it stays in the bag.
never been a fan of the F4 lenses. i always want the maximum versatility and to me not having the 2.8 is something i cant get past.
here is some stuff i shot (mostly as a 2nd) where the 24-70 and 70-200 is primarily featured:http://www.adamgierkephotography.com/data/pssites/PS_37/Current_Wedding_Gallery/index_12.html
and here is some more recent stuff where i have started to incorporate the 35mm a bit more:http://www.adamgierkephotography.com/data/pssites/PS_25/Wedding_Portfolio_12/index.html