People always say "I don't shoot a lot of low-light, so I won't miss the extra stop". What these people don't realize is the 24-105 is BETTER in all low-light situations that do not involve a fast moving subject.
The 3-stop IS on the 24-105 lets you hold the lens steady for 2 MORE stops then you would be able to with the 24-70. So in the same low-light conditions and shooting hand-held, the 24-105 will usually give you noticeable better results if your shutter speeds are in the 1/30 or slower range.
It's has a larger range and is much better in the hands then the 24-70. If you're not shooting fast moving subjects, then there's really no reason to have the 24-70.