One of the purposes of the new PDT file is to improve to speed to get the fully resolved POM. One of the downsides of the current pom model is that in case of multiple third-party artifacts with their own repository, Maven will always go top-down through the list of repositories. It would help if there's a reference to the location per artifact.
When the artifact is downloaded, Aether generates a _remote.repositories file, which contains the identifier of the repository when this artifact is coming from.
Mirrors were originally intended to redirect old repository URLs to new one. The '*' in case of a repository manager is actually a hack. A repository manager acts as a proxy, hence it might have been better to solve it like that.
When generating the PDT and when there's no '*', the mirror should be seen as a changed URL for the old repository, hence this should be used in the PDT as the repository URL.