This won't apply to everyone, but professionals such as myself would rather shoot with two bodies and two lenses so that a backup body is already with you.
I realize that for many professionals 2 bodies is standard / "best practice".
It does not apply to me personally. Especially when I want to have reach up to 400mm.
Never brought a 100-400 to a wedding.
I do not consider a 100-400 a "mega-zoom". To me a 4x telezoom is a quite benign, even conservative zoom-lens class that has been around for 20+ years.
My usage of a 100-400 would primarily be for all sorts of "outdoor sports/activities" and for all sorts of "wildlife" - from zoo to National Parks/Safari.
In additon to a few prime lenses I currently use 3 zooms (on 7D): 10-22, 17-55, 70-200/2.8. I do not consider purchase of a 400 prime lens. I want one additional, handholdable longer Tele-Zoom ... and 100-400 would be perfect. But I will stay clear of the current dinosaur Canon 100-400, which is outdated in every way - from its push-pull design to its image quality which is ok, but not great by today's standards and its 2-stops-at-best-IS whcih is substantially inadequate in 2013.
Basically I want a 100-400 II that matches my 70-200/2.8 L IS II in every respect. Build, sealing, IS, IQ - all the way to 400mm. At 400/5.6 it should be every bit as good as the current 400/5.6 ... and no, I do not believe this to be possible only in a 200-400/4.0 @ 12,000 Euro.
So, Canon .. get to work. It's time to deliver. :-)