The one area where I can understand people wanting to employ large areas of the screen is precisely that of the applications or "start" menu, mostly because when you navigate inside that you aren't usually wanting to look in there at the same time as look at something else, although there are obviously exceptions. For example, you might want to drag something out of such a menu onto the desktop to make a shortcut to an application. That would be difficult if the menu were full-screen. (It also appears to be difficult when Nautilus doesn't like the drag operation, but anyway.)
Here, GNOME 2.x and KDE 4.x are suboptimal just as GNOME 3 and Unity are presumably suboptimal. Indeed, as I have publicly noted previously, KDE 4.x has an annoying iPod-like start menu that seems to confuse people, all to be able to squeeze a menu into a ridiculously small portion of the screen. Clearly, that isn't a good solution, either.