The GUI uses the burst API, the API has public and private methods, the passphrase is only needed for private methods. You can remember it by using that checkbox. Currently you enter your passphrase, so that your accountID is known by wallet, so that balance etc. can be shown ... but you are right, that would also be possible, with accountID itself.

However, every programmer is able to write a new/alternative GUI for web or as application using just the provided burst API. (http://localhost:8125/test) So we may have different/better user interfaces in the future.