Those who don't want crappy wide end performance on FF??
(I got the 24-70 II 2.8, but I can see people going for 24-70 f/4 IS.)
If the 24-105 has "crappy" wide end performance, so does the 24-70 f/4.0, they are very similar optically.
Your 24-70 2.8 II of course puts both to shame!
Perhaps. I've never used the 24-70 f/4 IS but the MTF charts from Canon are much better at 24mm and the results at Lens Rentals were also better than the 24-105 L.
The 24-105L is widely panned at the wide end due to field curvature and softness around the edges/corners.
OK, I'll concede that the 24-70 f/4.0 gets the edge in edge sharpness and distortion at 24mm. However, by 35mm and 50mm the 24-105 is sharper edge-to-edge. At 70mm the 24-70 is sharper at the edges, but the 24-105 sharper in the center. at 105mm the 24-105 wins going away!
So, overall sharpness is a wash between the two lenses. The 24-70 is slightly better at both ends and the 24-105 better in the center focal lengths.
Unless you need its near macro capabilities or shoot primarily at 24mm there are no advantages to the considerably more expensive 24-70 f/4.0.
Maybe. I don't always find TDP matches what I've seen though. I trust Lens Rentals (where they also test like 50 copies of each lens) and Photozone more, although TDP has gotten better in recent years. One thing that can make results vary is whether a site refocuses for edges or not (and if not it makes the alignment problem very tricky; as for real world sometimes it tells the much truer picture and sometimes not) and how close the test target is.
24mm and near was always a very key zone for me with a general wide/standard zoom though since the long end is already handled superbly by 70-200/300 type lenses and getting something sharp near 24mm on FF was always a holy grail for zooms. That was always the real trick. Many could handle the other parts decently enough. The 24-70 II 2.8 finally does it at the wide end. It sounds like the 24-70 f/4 IS may more or less do it. If you don't care about 24mm, I'd just as soon stick with a cheap, light, fast 50mm and a 70-200/300 myself.
But yeah I guess it depends how much you care about the wide end or not.