Since 2.6 it seems that the distinction between the "stable" series and "unstable" series has pretty much disappeared. As someone who needs to keep up with current developments, this is often a real pain as it is like trying to build a house on quicksand. But one thing I didn't think of was that this increases compatibility; as evidenced by the model doing exactly the opposite ... in this post Andrew Morton suggests that suse may have caused two interfaces to the same thing due to releasing before something was accepted into the kernel.org sources.
So there's one advantage to having the moving releases as we do now -- everyone has no excuses not to keep pushing their stuff into the official trees.