Do you really think canon will let Tamron corner the market on wedding dslr videographers? no way! Just a matter of time before the IS comes out, I think within a year.
Are a lot of wedding videos shot at f/2.8? Is pixel-level sharpness critical for dSLR video? There are still the 24-105/4L IS and the 24-70/4L IS from Canon...
If you talk about whether you "need" certain aperatures and whether sharpness is "critical", we probably all would be happy with APS-C, STM motors, and f/4. But, when one is given an arguably better option for the same price, one will often go for the better option.
I actually did quite a bit of searching on this, and the answer is YES - the Tamron 24-70 VC appears very popular amongst wedding videographers given the commentary on various forums & youtube as it gives them both f/2.8 as well as image stabilization. The Canon 24-70 f/4 & 24-105 lack 2.8, while the Canon 24-70 f/2.8 lacks image stabilization.
If you were doing wedding photos+videos, would you rather carry a Canon 24-70 f/2.8 for photos *AND* a Canon 24-70 f/4 IS for video (on top of your 70-200 and everything else)? Or would you rather just carry a single Tamron 24-70 f/2.8 VC that can do both? Given the extra bulk of two lenses in the same focal range, I would see why many who do photos&videos are flocking to the Tamron.
So, if Canon continues to ignore this market for whatever the reason, they are bottom line likely going to lose significant marketshare amongst this professional group to Tamron. There is no good reason why they didn't launch an IS lens in the first place aside to make people buy the same lens yet again, as Tamron with much lesser resources and expertise has proved that it can be done with similar image quality. I am guessing Canon can exceed Tamron's quality, so if Canon had actually released the 24-70 II with IS, I have no doubts it would be very close to the current 24-70 II.
Again, I believe Canon will respond by the end of 2014 with a 24-70 IS, or risk continue losing customers to Tamron with one of their bread and butter lenses.