Until Firefox removed the location bar's RSS icon, that's exactly how I subscribed to feeds: click the icon and then click the "Subscribe in Google Reader" button that loaded. That might not have worked if I used a different client, though.
The real problem was that not many people cared to subscribe to feeds, so it was considered a waste of space to have on the toolbar by default. It's easy enough to add back though...
http://support.mozilla.org/en-US/kb/customize-navigation-but...
It's the de-facto standard.