See, the core premise of the original post is one that I agree with: an author doesn't owe their work to anyone except, perhaps, their agent and their publisher. Buying Rosemary and Rue doesn't somehow create a contract between us wherein I swear on penalty of death to do nothing but work on Toby books, all day, every day, until the series comes to a satisfying conclusion. For one thing, Kate would kill me. For another, if I worked only on Toby, with no pauses for other books, I'd go crazy, and the quality of the Toby books would decrease dramatically. And then The Agent would kill me (if Kate didn't get there first).
At the same time, the email which inspired the post contained a very different question. Is it wrong, the writer asked, to be annoyed when I read the blog of a favorite author and see nothing to tell me what the status of the next book is? And to that I have to say, quite honestly...
Look: there is no formal "deliver or die" contract between writer and reader, and there's a reason, as jimhines so helpfully pointed out, that very few publishers actually punish authors for missing their deadlines once in a while. Quality matters, and sometimes getting something done right takes longer than originally expected. I finished Late Eclipses in December of 2008, dammit! It was done! It was...nowhere near as good as it honestly needed to be, both to live up to the standards set by the first three books, and to live up to the standards I set for myself. I gave it to The Agent. She promptly gave it back, with a command to fix it. If I'd been working to a January deadline, I'm afraid my release date would have slipped more than a little as I took the book and ripped it apart to resolve its structural issues. Quality is always going to come first for me. Hopefully, it'll be a long time before that makes me miss a deadline, but even I and my OCD work habits can't guarantee that slippage will never occur.
At the same time, I do believe that there's a certain "social contract" which exists between writers and readers when those writers hang out their proverbial shingles out for the world to see. Once I've opened a professional blog and announced that hi, this is the professional blog of Seanan McGuire, come on in, I do owe you updates, even if those updates are things like "didn't work on Toby this week because I was busy following the Counting Crows around the Pacific Northwest" or "didn't finish the new chapter of Discount Armageddon because Alice got into the watercolors again." I have said, on some level, that I will keep you posted. The social contract demands that I uphold my end of the bargain, and if I don't, you have every right to get annoyed with me.
(This is similar to a scenario that plays out frequently with web comics, who have been dealing with their audiences online for longer than nigh anyone else. New guy hits the web comic scene, updating regularly. Sets an update schedule. Basks in the love. Starts missing updates. People start to complain. Snaps "I do this for free, and you should be grateful." Well...yes and no. I don't have the right to demand you work for me, but I do feel that, once you've entered into a social contract which says I'll get updates on days one, three, and five, I should get an update on those days, or, failing that, I should get information on why that update isn't there. That was the deal. If you tell me why the update is missing, you take away my license to bitch.)
In conclusion, no, George R. R. Martin is not your bitch, and no, you shouldn't view delays as personal attacks. Often, delays are there because the book is being made better. But yes, I do believe that once an author says "come hang out in my virtual office and play with my virtual fidget toys," you have a right to expect to be told what's going on, and a right to ask "why is the eighth book in this series not out yet?"
It's all a matter of where you stand.