I know it's deterministic, and that's great, but I prefer using the text from 12 word child seeds as passphrases instead.
Imagine trying to write this down, by hand:
AwMJYOqM+ibwWt/s4BCcH
Now, imagine writing this:
core private bacon derive phone soon baby gun main local bench you
I love the simplicity of 12 words as easily readable text.
However, there are services that will require you to register your passwords with specific criteria: lowercase and uppercase letters, special characters, and so on... In that case, what do you do? I use KeepassXC or Keepass2 to manage that for me.
You're absolutely right, that's a valid use for it.
I just fear that some folks will use it as a passphrase to secure their Bitcoin, and have it lead to disaster in the future.
They're either not going to back up their passphrase at all, or they'll make a mistake while writing it down, because writing down AwMJYOqM+ibwWt/s4BCcH opens up too many possibilities for errors.
But, sadly, most folks aren't going to realize they can use BIP85 child seeds as passphrases, because they're so stuck on the idea that
it's a seed. Yeah, it's a seed, but it's also just text. Use it for any purpose.
The beauty of using BIP85 child seeds as plaintext passphrases is that if you also use a hardware wallet that generates QR codes, you can easily generate & scan your passphrase.
I use Krux for this. In fact, I use 2 Krux hardware wallets.
Krux A: Load the parent seed. Generate a 24 word child seed.
Krux B: Load the parent seed. Generate a 12 word child seed & display it as a plaintext QR.
Krux A: "Add a passphrase." Scan the QR on Krux B.
There's a SeedSigner fork that adds passphrase QR and display QR of a plaintext mnemonic (among other features), but I don't own 2 SeedSigners, so I haven't tried it yet. I tested it with one SeedSigner & a Krux. It's great.
That workflow goes like this:
SeedSigner: Load parent seed. Generate a 24 word child seed. Go back to the parent seed and generate a 12 word child seed.
Krux: Scan the 24 word child seed QR from SeedSigner. Add a passphrase by scanning the 12 word child seed as a plaintext QR on the SeedSigner.