As near as I can tell, this letter constitutes a new defense from the Bush administration. See, Clarke once said something nice about George W. Bush, which means that any criticism is completely negated. Clarke thought Bush provided strong leadership on 9/11, which means that Clarke cannot possibly think that anything Bush did or didn't do prior to or after 9/11 was bad.
I think a defense like this could have far reaching implications! Forget about divorcing for irreconcilable differences! Because you loved the person you married on your wedding day, it negates any future criticism, even when you catch your husband snogging with your best friend. You loved him once, which means your feelings and opinions can't ever change! Nyah, nyah, nyah, nyah, nyah, nyah!