Earlier quoted context omitted.
> in our heads So it's short enough to remember and likely has some sort of pattern. There's a limit to what a person can remember, lower if there are several people that have to remember it.
It is not very difficult to memorize random strings of arbitrary characters. I use a password manager to manage most of my accounts, but the important ones, like banks and email, I keep in my head. I use my password manager to generate a 15 character string of alpha+numeric+symbols. The symbols would kind of make it hard, except that in my head they are just upper-case numbers, mostly (shift-7, not ampersand). And in…
But for x many customer credit card details you're really looking for a much longer password that that. I'm talking 64 characters or more of pure random data.
You shouldn't be compromising for the convenience of being able to remember a password when it secures such critical data in my opinion.
Edit: I do agree though that your method is a very good way of remembering password.