The timing should not take away from his message, nor should it impact how important his message is. I have a feeling you will not see this reprinted very widely as it is, since it's an unpopular view from a popular author, as it is. I'm not saying you should e-mail the letter to everybody you know, but at least file it away for a rainy day or an argument with a revisionist.
Considering he wrote this just a few years before he died, I get the feeling, from what I've read, that he knew he didn't have too much longer, and he had a lot of unfinished things to do do. Getting into debates and/or arguments over his views over the atomic bombs (which were very firm, even after 50 years had passed) was probably a waste of time to him.
Would you want to spend the last few years of your life arguing with people over something which you most definitely would not change your mind? It may sound like I'm defending him, but I believe I understand where he was coming from when he chose to have this published after his death.