The URL bar serves as the search bar as well, and for most people that's their primary mode of navigation. Even if they don't really care about URLs, I don't see anyone wanting to add another step every time they do a search.
Ctrl + K gets you into search mode only. I find it annoying sometimes when what I'm searching is similar to a URL in my history, so it's useful to specify it's a search with a "?" before a query.
Nice shortcut, I always did F6 since IE6 time. It seems to work with all browser too :)