On the contrary, I think having an Ajax posting interface would be immensely valuable. Allow me to illustrate.
I was posting the other day, and my browser crapped out and said "Server not found." I hit refresh, and the site was indeed there (just a random glitch, or perhaps my internet connection hiccuped for a second). But my post was gone. I hit "back," and my post was still gone. History.
Today, I was using Gmail, but wasn't connected to the Internet. I wrote my message and hit "send", not remembering that I wasn't connected. Gmail noticed, and told me I was offline without losing the message. This is built in via Ajax. I connected, retried, and my message sent.
I'm sure the lost post scenario has happened thousands of times to thousands of people - every one of us has lost a post, no doubt. Even if you save drafts frequently, you're still relying on an HTML form submission to save your post, which can be messed up by a number of things. Ajax-ifying the WP write interface would allow WP to confirm that a connection is available, send the post data, and only reload the page after it received confirmation that the post was saved. It may or may not be faster (e.g., compare gmail to hotmail), but it would be a lot more reliable.