While the majority probably does, I don’t maximize anything that doesn’t have subpanels by default (like IDEs). In particular, I generally size application windows such that their main text content (if any) takes up a suitable middle column on the screen. That also means that I often have application windows with fixed-sized side panels not fill the whole width of the screen. My browser windows are by default something between 5:4 and 4:3-sized. Even with vertical tabs, the added width wouldn’t be enough to make them full-width.