Most of my weddings are outside and I use a fill flash a lot. I'm usually shooting between 4.0 and 11, just to make sure I get things in focus. For inside shots, 2.8 would very rarely be fast enough, and even if it was, it's hard to hand hold a 24-70 and get sharp pics at less than a 30th of a second. On the other hand I have got sharp shots with the 24-105 IS at 1/8 of a second. I usually use a 35 1.4 for the low light stuff. I guess you need to ask yourself, how often you will, in acual practice, shoot at 2.8, because depth of field is pretty shallow at that aperature and not getting something in focus at a wedding, isn't exactly the easy to fix in photoshop. I think most people get a 2.8 lens, because they think they will shoot it at that all the time, but it's usually not the case, specially at weddings. Fast primes are a much better alternative to low light, than 2.8 zooms.

although it is sharper than the Canon 24-70 but then the 24-70 has the f/2.8 all the way trough the range. 
