[atondwal]

Consider an ASCII password of unknown length.

Now you just show your mark each symbol to check if it's a part of the password, which would drastically and usefully reduce your search space (unless it a password that uses almost all ASCII characters, but those are extremely rare...).

[PeterisP]

No, the proposed method can't check if it's a part of the password, it can check if it's a part of a password/something the person has ever known. All alphanumerics would be included naturally.

The reason for PIN's is that if your pin is '8243', then that number will provoke a "recognition" response much different than, say, '8244' which (to you) is just a random number with no specific associations.