Though it depends on how you use your screen: if you like to tile windows horizontally, it takes more of your space. Tree Style Tab has a keyboard shortcut, F1 by default, to toggle its sidebar panel, and when I’m tiling side-by-side I often use it to close the tab bar for the moment. I’ve been surprised just how many pages are at least a little broken at 618px wide (what I get for two windows side-by-side, with the sidebar open)—I expected it to be rare, but while it’s not the norm, it’s hardly rare.