Please note the date of the article - it may not be current nor does it necessarily reflect the author's current opinion on the matter. See this comment.
The ability to fork is a wonderful thing.
In the open source community, the ability to fork software projects is a wonderful thing, as it allows taking a software snapshot in a completely different direction from what was intended by its current maintainers.
Projects get forked for reasons that can be categorized in political (changing ownership rights, controversial decisions made by the project maintainers, etc.), technology related (where maintainers disagree about the direction of development and implementation) and personal.
Forking is a bad thing.
Wait... did you not just say forking was wonderful?
The ability to fork is wonderful, as it gives great power to the community. But forking itself is bad for the project, as it results in two projects with weaker development and support, a weakened potential to grow and a divided and confused user base. It leads not only to separate code bases, but also to a divided developer and user community and should be considered last resort.
In the best case scenario, forking is choosing the lesser evil.
No matter how much effort is put into collaboration between the fork and the original project, in the end it always ends in lack of compatibility and refusal to provide support to confused users in the different camp. This is why the Backdrop creators' reassuring statements about cross contribution should be taken with a grain of salt.