"Fair enough, but can Red Hat guarantee that all the bug fixes present in package version N+1 are backported to version N, which happens to be shipped in RHEL 6 ?"
That is never the goal. Just like new features, bug fixes are also cherry picked. Not everything gets the same priority. Upstream projects often exchange one set of known bugs with another set of unknown issues in newer releases. So just moving to a new upstream version doesn't solve the problem.