Steve, while a few mm on the long end doesn't seem like much, a few on the wide end can be huge. With the digital crop factor your 28mm lens is closer to 42 in performance - not very wide at all. I know that I use the wide open end of my 17-55 enough to make me seriously consider a 12-24. I don't expect to use it nearly as much as the 17-55 so, like I said above, I can't justify the price of a Nikon 12-24 but I do think it would be quite useful in many situations.
Phil