Note that the "additional phrases" and the nonce is an optional values, which means that you could only generate the same seed phrases with the same email and password.
Which would be highly insecure.
Backup option is provided in case you forgot your email or password, or any other values you have used and it is recommended if you tend to store valuable crypto.
Which is entirely my point. Given that you
should be backing up your wallet recovery process offline, then why not generate your seed phrase securely and back that up, than risk generating it insecurely by using this method and then backing up your additional phrases and so on (which also don't have a checksum or any way to identify an error).
Of course, you could generate random phrases via the website which is generated by the browser cryptography API, and make an encrypted backup of it
https://bip39.org/#mnemonic.
I wouldn't recommend that anyone uses a website based javascript generator:
https://bitcointalk.org/index.php?topic=5324030.msg56590276#msg56590276Yes, as I have said earlier "use it on your own risk" and my recommendation is to store crypto on something more audited and well used. This website is only at the proof-of-concept stage for those who seek for easy binding of the mnemonic against something easy to remember. My idea to publish the website here is to gather any ideas that could lead to improvement and get the audit done. If the function is mature enough, will port to other languages like C or Rust for secure key generation framework. JS is preferred since it could demonstrate the whole process on Node.js and Web Browsers without sacrificing much time for building an UI.