For me, the primary intent of the 24-105 focal length range is a general purpose, walk around lens. Put it on and off you go, not carrying any other lens. It could be touring, family times, whatever. The red ring gives you the comfort zone of “high quality” and good weather sealing.
Scenario – you are in Italy, on a one hour quick walk. The doors are open in this old Church, in you go. It is spectacular, marble, gilt work, not well lit and, you are virtually the only one there. The 24-105 will do the job. The f4 is not really a limitation, the IS gives you what you need. A few minutes later a women climbs the steps to the organ and begins practising for tomorrows service. There are massive organ pipes on either side of the Church, true stereo (old time) sound resonating. OK, eye contact then climb the steps and do what our cameras can do, switch to video and record this “happening”. Now the Camera and Lens combination is being asked to record video in a dimly lit space – the IS becomes critical. The width 24 sets the scene, the reach follows fingers on the keyboard and feet on the pedals.
So, in the scenario above, if the 24-70 2.8 II had been the chosen lens, I know the whole Organ scenario would have been compromised. The flexibility to go video in a “walk around” scenario is handicapped. Canon, why?
OK, our new 24-105 II. Seems to have it right for the intended purpose. A little extra weight, no real problem, we DSLR people, have made a choice to carry larger objects than point and shoot. Better IS, right thing to do. Sharpness – as our camera bodies evolve they resolve more. So, lenses need to follow suit, they need to resolve more to mate with camera bodies that exist today and over the next 8-10 years. This is a refresh intended (I would assume) to last thru maybe 3 generations of camera bodies to come. Canon, why?
For me, Canon has come close on both of the above lenses but left us with dilemmas (opportunities for other manufacturers) on both. Why?