It all depends on the subject matter. Having said that, its not really the focal length, but the field of view that we are talking about, which boils down to composition. I use the 12-24 quite a bit. However, lately I acquired a FA31Ltd and was experimenting with it. In doing so, I stumbled across some details in a landscape that I had not really been looking at, since I was looking at the large picture of the landscape and not necessarily the individual details.
One instant that got me started in wide angle landscapes was up in Alaska. How does one even start to convey the immense size and wide expanse of the views up there?
So back to the question of how wide? One of my best pictures was taken with the 10-17FE at 10mm because I needed the entire 180 degree field of view in one individual shot as everything was in motion (two ships at sea, 100 feet apart performing an underway refueling) sailing into the sunset. This could not be stitched, and a rectilinear 12-24 at its widest is 100 degrees - or in this case 80 degrees (of half the picture) too narrow. But that need is not all the time - its really using the camera to capture a story - and what you want the story to convey to the observer.
So some shots will require - as wide as possible others a bit more tailoring. Most of us can and should use the lenses we have more effectively (but new toys are so much more fun). You can use your standard 18-55 at 18 and stitch for additional width. There are additional ways to work around the problem. I was just looking at some babbling brooks that are essentially small waterfalls in portrait mode - the waterfall literally coming right in to your lap - a very effective use of the foreground.
I voted for 12 and up since I like that segment - however, with this new lens - the 31, I am gaining a new appreciation for the shorter and narrower field of views and what they have to offer....