I can understand why he wants to be on the bottom of things, but I don't get why encouraging people to use postal mail rather than email is supposed to help. Email is just a medium, and it's a much more sensible way to move information around. The widespread habit of checking email every 5 seconds is by no means compulsory. I wonder why Knuth says that the batch mode communication he wants to do cannot be done properly with email.
I think it's just that he's trying to raise the barrier to entry a little higher. If you know you'll get a response in a few days or weeks on paper, instead of a day or two later in your email client, that makes you look for other, quicker ways to get the answer you're looking for, which reduces the load on Knuth (and his secretary).
I can understand why he wants to be on the bottom of things, but I don't get why encouraging people to use postal mail rather than email is supposed to help. Email is just a medium, and it's a much more sensible way to move information around. The widespread habit of checking email every 5 seconds is by no means compulsory. I wonder why Knuth says that the batch mode communication he wants to do cannot be done properly with email.