Sure, MultiMarkdown is awesome. It's the cornerstone of 90% of my written words. But have you ever tried to work with MMD using multiple AppStore apps? It can be frustrating. Jumping between apps usually means closing the document, quitting the app and reopening the document in the new app. It also means dealing with embedded images in surprising and disappointing new ways each time.
Brett Terpstra and The Soulmen did what good people usually do. They worked together to make something better for everyone. The TextBundle format isn't a declaration or proprietary file format. It's a guide to help publishers of AppStore apps work with Markdown in a dynamic way. It makes sense for users and for apps.
TextBundle is directed squarely at sandboxed apps and how they can communicate. This will work better if everyone writing AppStore apps for Markdown gets on board. I'd buy an app that supported this format as a feature.