For use, kind of a lark lens really. Getting super close to stuff for more interesting perspectives. I've also seen some pretty neat nighttime landscapes / starscapes done with ultra wides. Ken Rockwell also has some neat stuff on his site that I'd like to try.
http://www.kenrockwell.com/tech/how-to-use-ultra-wide-lenses.htmThe fact that it's not a "must have" for me is why I'm looking outside of Nikon lenses. Just can't justify the Nikon price for a lens I won't use a ton (or who knows, maybe I will). I put the major chunk of my lens money into my Nikon 18-200, which is the lens I use 75% of the time. I have Nikon 35MM, but the Nikon prime lenses (I have a DX) are so freakin' cheap there was no reason not to go brand with it. My third lens is a Sigma 150-500, which I was leery about at first, but is actually a really nice lens, especially for the price, and why I wouldn't hesitate to get another proven non-Nikon lens, especially for limited use lenses.