There are plenty of topics discussing the 70-300L, but they are from the pre-5DmkIII era. I've been thinking of getting a nice tele lens to complement my kit 24-105 f/4, and I cannot find a single reason not to choose the 70-300L. It will be a lens that is going to be used during in field action during walks, political meetings, etc, so hauling the 70-200 f/2.8 mk II around is going to be tiresome, and it is widely regarded that 70-300's IQ is similar to that of the 70-200 f/4 IS, while still having 100mm extra.
Since 5DmkIII is here and slower aperture problems might be overcome with higher ISO numbers, is there a reason not to choose the 70-300L? Thank you.