These APIs were often limited, though, not exposing all of the features of a publishing platform, and making it difficult or impossible to manage other aspects of a site like embedding content, modifying design, or managing comments.
The lost infrastructure of social media.
Anil Dash

Yeah. The saddest thing about the proliferation of proprietary clients is that Atom API was specifically devised for 3rd party extension while maintaining interoperability. And while Atom API was extended by some as part of internal publishing workflow tools, this capability was never really adopted as part of the current social web.