* Currently, when I'm done with a document, I can safely press Ctrl+W, and the right thing will happen (the tab will close, or the window will close). If I had to press a different shortcut to close a window, I'd sometimes accidentally press the shortcut to close a window without realizing that I had other tabs open.
* Blanking a tab isn't an especially useful action.
* For people who usually use windows rather than tabs, it's nice to have an easy-to-press shortcut (Ctrl+W) to close the window.
==Should this be a preference?==