The company I work for has used it as a relatively simple method for implementing mutual TLS (mTLS) for legacy apps or systems for which it would otherwise be annoying or more difficult to integrate mTLS for, or which doesn’t support mTLS with custom trust store.

same here. This thing is gold for "80% solutions" in that respect. It's easier to sanely integrate with legacy transport protocols than trying to update the legacy code base to implement mutual trust the harder, more direct and more error-prone way, IMO.