A huge number of Chrome users go to google.com to search for things; if the new page looks like the Google search page, and it moves your cursor to the address bar, Google hopes these users will learn that you can just search from there. Maybe it's an experiment that will fail, but it seems like it might work.
In Firefox at least, I've learned that the regexp isn't as fast if you type a search query in the URL bar. It takes slightly longer for the browser to decide if the text is a URL or not. That's why I reenabled the search bar in Firefox (it's not required).
I had the same problem which forced me to enable the search bar. But I've since discovered that if the text you entered into the search bar starts with a "?" character, Firefox immediately assumes it is a search query.