This post from Melissa Coates is a good example of not thinking through your product architecture early on. The short version of the post is that Power Bi Desktop is a better place, perhaps really the only place, to author your Power BI Reports. Melissa covers a few potential issues while using the Power BI online editor, but the big item is that you cannot easily prevent conflicts and track versions there.
Power BI started as the online service, with the Power BI Desktop tool seeming like a bit of an afterthought. At least from the marketing perspective as all of the early demos and media from Microsoft was about the online report service. Since then it's evolved to become the primary way we will build reports in SQL Server moving forward, which I think is a good move.
While I can understand the developers at Microsoft not really thinking that the service would on-premises and maybe not considering the need to provide some text format for the reports, I can't understand why they didn't learn from the Integration Services team and realize that binary versions of programmable items don't make sense. We need a format that can be easily versioned, and maybe more importantly, stored in a VCS and diff'd in a way humans can understand.
When building a format for storing code, please consider the need to work in a team and version the changes made. This means any format should not be a) binary and b) difficult to decode. Separate visual elements from logical elements and ensure a text version of this can be examined by developers. You can use XML, JSON, YAML, or any other text based format, but choose something that makes sense. Even if you add your own extension, ensure that standard tools that work with code can use this.
I do know the PBIX format is a ZIP file, but zip files don't easily integrate with a VCS. We could use hooks to extract/rebuild out files on commit/checkout, but that's cumbersome and silly. I'd rather that the PBIX were a folder with the files inside. Users, including my Mom, can zip a folder and email that if needed. To me, that would have been a better structure from the beginning.
Microsoft is supposed to be a company providing platforms that we build upon and use in our work. The decisions for the Power BI service seem to be poorly thought through with that in mind. I'd urge them to create a baseline set of rules for future products that consider DevOps, teams, and the need to track code.