Think about it? How many L series lenses actually have IS? The mkI 24-70 was my first piece of L glass, and before buying it I went back and forth between the 24-105 and the 24-70, debating whether I wanted 2.8 or IS. When it came down to it, I decided to just turn off the IS on the kit lens and see what happened. No IS wasn't a real problem until I got real low in SS. It got me to thinking, really, when am I going to be shooting people, or really anything at all under 1/10th of a second without a tripod? So I went with the 24-70 and haven't missed IS one bit. I became confident enough without IS that i also bought a 10-22 no IS, and the 70-200 2.8 no IS (and mind you, I'm shooting on a 7D, so the 70-200 is effectively a 110-320mm)...
So in short, the only short-coming I can see with the mk11 is the change in filter thread size which puts someone with 2 other lenses at 77mm thread in a pickle for filter investment (maybe I need to get a step up ring, and make the investment in 82mm filters?).
6 Zooms and 11 Primes. Not a small number. Secondly IS isn't just for shorter exposure times. It works fantastically to reduce up/down movements when taking panning shots at any focal length.
Canon EF 24-105mm f/4 L IS USM
Canon EF 28-300mm f/3.5-5.6 L IS USM
Canon EF 70-200mm f/4.0 L IS USM
Canon EF 70-200mm f/2.8 L IS II USM
Canon EF 70-300mm f/4-5.6 L IS USM
Canon EF 100-400mm f/4.5-5.6 L IS USM
Canon EF 100mm f/2.8 L IS USM Macro
Canon EF 200mm f/2.0 L IS USM
Canon EF 300mm f/4.0 L IS USM
Canon EF 300mm f/2.8 L IS II USM
Canon EF 400mm f/4.0 DO IS USM Lens
Canon EF 400mm f/2.8 L IS II USM
Canon EF 500mm f/4.0 L IS II USM
Canon EF 500mm f/4.0 L IS USM
Canon EF 600mm f/4.0 L IS II USM
Canon EF 600mm f/4.0 L IS USM
Canon EF 800mm f/5.6 L IS USM