Justine,
Often a photographer's mindset automatically turns to UWA focal lengths when it comes to landscape photography. Sometimes using UWA lenses is not a great idea, especially if your desire is to get the greatest coverage left to right. The UWA lens will often result in a large area of sky and a large area of uninteresting foreground with a small strip of interest somewhere in between.
Although we see a large expanse when looking at any vista, our eyes have selective vision and can concentrate on the interesting portion of what we see. The camera doesn't have this selective vision. When shooting landscapes, I like to concentrate on specific interesting images within the landscape and thus will often use a longer lens. If using a UWA lens, I like to concentrate on an interesting object (rock, plant, etc) in the foreground to achor the image and to provide an illusion of depth.
I used a 24-70mm f/2.8L lens on a 1.6x camera (38.4-112mm equivalent) for this shot of Yosemite.
If I had used an UWA lens, I would have captured a broad expanse with a tiny little waterfall.
I used a wide lens for this image but, my main subject was the lava rock formations in the foreground. The tree just balanced the shot and the lava rock formed leading lines bringing my attention to the tree.
If you need a wider left to right coverage, a several shot pano is easy to shoot (even hand-held) and is equally as easy to composite using an editing program like Photoshop Elements or Photoshop. If you have a tripod with you, shooting with a longer lens with the camera in the portrait position will provide wider top to bottom coverage. Left to right coverage is predicated on the number of images you composite into the pano...
BTW: There is one advantage to the Tokina 12-24mm lens over the 11-16mm. The 12-24mm "can" be used as a "normal range" zoom since its 24mm side has a 38.4mm equivalent. I brought my 12-24mm Tokina to China to use if I needed a wide shot. However, I also thought that I could use it for my "normal" zoom; if the 17-55mm f/2. IS lens died while I was in China...
OTOH, the 11-16mm is 1-mm wider which can be important. It also has a constant f/2.8 aperture which is one stop faster than the 12-24mm f/4. However, I most often shoot my UWA images at smaller apertures to gain the widest DOF. Another advantage of the 12-24 models over the 11-16mm lenses is the lower price...