Earlier quoted context omitted.
I don't get why people think this is an editor feature, and not an OS feature. When I want to view two files at the same time, I open two windows, and put them side by side. Not everything needs to be fullscreen all-the-time. Let the OS manage windows. It's better at it! Besides, having windows automatically become half-maximized is easy with the right tools, like BetterTouchTool (Do it with a gesture!), or Divvy.
The OS isn't better at it if there is none or limited ability to manipulate the windows in a keyboard driven way. I use and love the 3rd party tool SizeUp, but SublimeText2's integrated pane management (including moving tabs between them) is far better than plain windows.
Want to view two files? Ctrl-right on one, it's now on the right side of the screen.
Command-O, chose the new file, Ctrl-left on it, it's now on the left side.
I'm honestly very curious what Sublime/etc offer that is better than what you can do with the OS?