Does that mean for multi-version or TV discs we will need to have multiple copies or go back to using separate MKVs? If that is a case this seems to be a premature change until stream selection is possible.
For example: I Am Legend has both the Theatrical and the Alternate Ending versions. By linking to separate .mpls files you can have one copy of the disc with both versions avaialble to choose from.
Edit: With this change (if I am understanding it correctly), it will complicate title selection. Right now you can link the individual .mpls files and title them so you can tell what they are. With a generic title selection process all you will see at most is Title1, Title2, Title3, with length. That will be an issue if you want a specific special feature and can't tell what is what. Also going to be an issue for TV series discs since the titles are not always in actual episode order. For example a disc may contain episodes 5, 6, 7, and 8 but by title number order on disc they are 8, 5, 7, 6.