I have moved instances, so it made a practical difference to me. I also follow a lot of people who have moved instances and in all cases I only noticed because they posted about it (and hence there are probably more who have moved that I didn't notice).

It needs improvements, such as the ability to move in spite of the old server (the ability to get at your private key so you or the new server can sign an assertion about the move with the old accounts key would be one option which would at least give people the choice of being prepared), and moving over the old posts (there are some stupid concerns about "faking timelines" which you can trivially do anyway, so I think this is a question of time to resolve that - it's not technically hard), but it already does offer significantly more freedom than Twitter.