DOF control is important but i see myself using this particular lens for group shotsIf used for group shots, the 24-105/4 is perfectly good. Many group shots require f/4 or smaller anyway. But occasionally a small group is in the same plane of focus and can be shot at f/2.8 - f/3.5 (or even f/2 with a prime).
However, for general use at weddings, I much prefer the new 24-70/2.8 II — in addition to primes. It is more compact and overall better than the original version. F/2.8 allows for much less use of flash than an f/4 zoom. An f/2.8 zoom will also focus better indoors than an f/4 zoom.
that being said, i would not trust anything slower than a 2.8 in really low light situations so any of the F4 glass wont make it into my bag. 2.8 just lets you see better and auto focus seems to benefit from the extra stop of brightness.Good points!
the last thing i would point out is that on a full frame camera the difference between 70mm and 105mm is minimal imo. if you are concerned about lack of reach on full frame for weddings then you really should be looking at the 70-200mm 2.8. its an absolute star performer!