You're right, and I suppose that's why Twitter doesn't add it. But when you write a thread and then spot a typo right in the middle, it's a disaster. You don't have to deal with clunky Twitter UI but also have to recreate the content (images, text).

Chirr does not update the content but removes the edited part of the thread and republishes it, so it's impossible to abuse.

Another thing I working on is delayed publishing ala Gmail.