I have the 5DIII, the Zeiss 21mm and the 24-70mm II. When I go hiking or otherwise off for landscape photography, I only take the Zeiss 21mm and the 70-200mm F2.8 II. I have little use for the 24-70mm range in landscape (the 24mm end is good, but 21mm is usually better). Generally, I either want to see all of everything, or I want to see things far away up close.
The Zeiss lens itself is fantastic. In most cases, I just set the focus to infinity (it has a hard stop which is pretty much on in the summer for infinity, in the winter I roll the focus back just a tad back to get proper infinity). It's a nice, durable lens with great optics. If you specifically need to focus closer, you can use live view to get precise focus if you need it. For landscape, it just means you're a little more methodical, but you're rarely in a hurry in these situations. It has such a wide depth of field though, if you need to, you can just focus close enough by quickly eyeballing the scene.
Since you already have the 24-70mm I, I think you'd get more bang for your buck adding the Zeiss, instead of upgrading to the 24-70mm II. Here are some shots from my trip to Utah a couple months ago:Deadhorse Canyon 2
, on FlickrDeadhorse Canyon 1
, on FlickrBowtie Arch
, on Flickr