Anyone who is arguing against or dismissing IS, is IMO, simply rationalizing the purchase of a lens for thousands that lacks said feature. And that is fine.
But lets be real, IS is highly desirable. There is no disadvantage with IS except a *slight* cost increase, and lets be frank when you are spending $2200 on a lens you can't really argue costs. The 17-55 has IS, the 70-200 has IS, the 24, 28, 35mm consumer primes all have IS. Obviously it is very doable both financially and engineering wise.
The advantages with IS are plentiful. Less shots needing tripod, less blurred shots, and even if you have a steady hand your shots will be slightly sharper with IS due to its nature as no one has the hands of a tripod. If we were all as steady as a tripod, tripods would not exist. More possibilities for photos are unlocked. And it helps video, too, if you are into it.
There is no effective argument against IS. Canon is just playing marketing games getting people to buy the same lens over and over again, because they can. That is why it is not in the 24-70 II, no other reason. When the 24-70 IS comes out, the 24-70 II will drop like a rock in value. Take a look at how much less resale the 70-200 non-IS versions have versus the 70-200 IS versions.
So, while people can say they might not need it, that is probably true. But you also don't need L lenses or a full frame camera. IS is another tool in the toolbox that is HIGHLY DESIRABLE. Let's just hope we don't have to wait too long for Canon to milk the non-IS version before the inevitable 24-70 IS release.
Yes, the 24-70 II is sharper than the 24-105. But that has nothing to do with IS, it simply uses better glass. Once the 24-70 f/2.8 IS comes out, the 24-70 f/2.8 will be soundly outclassed and lessened in value IMO.
The problem is that the optical formula with IS is different than the one without it. Are the 70-200 f/2.8 IS versus non IS the same optical design? According to the rumors circulating before the 24-70 II came out, both the 24-70 II and a 24-70 IS variant were being field tested. They were different lenses. The testers preferred what became the 24-70 II. If that is true, then it is harder to design something that is as good as the 24-70 II and has IS.