While I'm sure it adds a lot of usability features compared to the default Emacs config and it looks pretty cool, it's nowhere near as immediately accessible as Atom or Sublime - for example, I have no idea how to close an open file without Googling whereas in Sublime or Atom it's either click the "X" or use the standard Cmd-W shortcut. I also have no idea how to open a project or search for a file, and don't really know where to start discovering those things, whereas in both Sublime and Atom you can browse through the menus, or hopefully quickly discover the Cmd-P command launcher which lets you type a command.
If I wasn't somewhat familiar with Vim, I would have absolutely no idea what was going on as by default it uses Vim's modal keybindings. I also noticed an annoying lag when pressing spacebar to bring up the command list thing - slower than any lag in Atom!
It does look intriguing and I'd love to learn to use it more, but I don't think you can really say the usability of Atom/Sublime and of Spacemacs to a new user are anywhere near equal, never mind "how would they justify their choice?"!