In Vim there are many benefits from having multiple buffers open within the same Vim instance. Buffers share session data of the Vim instance they're in. This has a lot of different benefits, including automatically sharing whatever options or settings have been set (many dynamically or temporarily set), sharing of Vim registers, macros, and command history, all of which are heavily used when editing (if OS handled they would generally share a single clipboard register), Vim-controlled navigation between buffers and windows, Vim-controlled scripting between multiple buffers, Vim-controlled searches of multiple buffers, and more.

Because there are so many benefits of having multiple buffers/files open within the same Vim instance it makes sense to have multiple buffers viewable at once, not to have only one viewable at a time. You lose a lot of editing power if buffers are not open in same Vim instance; it doesn't make sense to open a new file separately in different Vim instance. Moreover, many times split windows are useful to show two different views of the _same_ buffer at once.

Because then I've gotta deal with two windows. Cmd+~ : One's now in the background, the other isn't. Switch apps, one comes back, the other doesn't. Switch monitors, the sizing's off. Try to do anything other than 50/50 split, have to adjust manually. Try to use TM's project browser (seriously), doesn't work. Try to swap that second file for a different one, have to close the window, open a new window & reposition. It's generally a much clunkier workflow.

Granted, I only know that from using VIM (and occasionally SublimeText), but it's been one of my "Dear GOD Give Me This in TM" requests for a while. I've got SizeUp and love it - truly one of my must-have apps - but to me it's a hack, not a solution.

What's the key to add a third column and have the others automatically resized?

Do other windows start inheriting the project drawer if older windows are closed?

Each window has multiple tabs - how do you tear off and replant them using the keyboard?

Doh, now the temporary horizontal split panes like find-in-project, the python console, etc, must be attached to a specific narrow window.

And so on. I like a multi window desktop and am certainly not a maximise-everything guy. But relying on the OS for everything is not optimal.