Why allow invalid Mnemonics?

[pezcore]

Why does Mnemonic::parse_in_normalized_without_checksum_check exist? Isn't the main usefulness of this crate the fact that Mnemonic instances are guaranteed to be valid? Now if someone writes an application that presents a Mnemonic to a user, they have to manually trace the source of its initialization and make sure it wasn't created with parse_in_normalized_without_checksum_check or else they risk presenting an invalid mnemonic.

It makes sense for possibly-invalid mnemonics and operations thereof to be in scope of this crate, but I feel like their implementation shouldn't break the checksum validity invariant of Mnemonic. That way, application authors can simply trust that Mnemonic instances are valid.