The problem is that Google hasn’t made it very easy to download these fonts for use on the desktop. For designers, that makes life harder — unless you begin your design in the browser, you need those fonts available in programs like Photoshop when you’re creating your design comps.
Fortunately, Joe Maller has collected the fonts made them available for download in a tarball. If you want to use Google Web Fonts but don’t want to abandon your Photoshop mockup process, head over right away.
Of course, it could be worse. Users of services such as Typekit pay an annual subscription for access to web fonts, and still have to make a separate purchase that can easily cost hundreds of dollars just to use a couple of those fonts in Photoshop.