>A fraction of a second delay probably wouldn't be noticeable to the end user
It is (try it), and retries/outages would be very noticeable. One of the applications I'm particularly interested in is directory services, and as you can imagine browsing through trees is extremely sensitive to responsiveness. Dialing a name should also be near instantaneous.
>DNS may be distributed but that's part of the problem - you can't just set your address to redirect to another number when you're out on lunch
You can (.tel allows you to switch between "profiles").
>it'd take too long for the changes to propagate through the system.
You can set TTLs to whatever you like (60 by default).