The atom feed will represent the compiled and rendered end content. If I have, say, source code snippets, that leaves me in a position of needing to parse out the original code so I can redo it in whatever new highlighting syntax my new blog uses. I might end up with resized images instead of originals, etc etc etc. It's not a real substitute for a proper backend export.

This is why I'm such a huge fan of Octopress and nothing has been able to pull me away from it. Everything's in a git repo. Backups are one command. All my original text is safe in Markdown format.

It might not look as cool as Medium or Svbtle, or have all the features, but it's relatively easy to use, and more importantly immune to platform lock-in.