I remember when I was first exposed to Firefox. A friend showed it to me, pointing out that it had tabs. I didn't like it; after all, the taskbar is like a set of universal tabs -- why does your browser need its own taskbar?
I still sometimes wonder that. Why are the browser tabs so much better than the OS taskbar's application buttons? Isn't there a good way to merge these?
In some cases, such as browser tabs, yes I think they could be merged. I even wish they would be merged. It would standardize the tab system across all applications, prevent problems such as apps which don't support ctrl+F4 to close a tab (Notepad++) or which don't support middle-clicking a tab to close it (Lotus Notes). It might even make app makers' jobs easier, because they wouldn't need to implement their own tab bar; they'd just let the OS handle it.
But in other cases, this wouldn't work, for example where there are multiple sets of tabs. Eclipse, for example, has several window areas on the screen, each of which has its own tab bar. This is incredibly useful, and would be stifled if it were to be forced into one single tab row. In this case, a standardized tab system simply wouldn't work.
Unfortunately this is something of a chicken-and-egg problem; the OS makers and the application makers would need to work together to make one big change, and those types of things just don't happen often.
Do you ever think of your taskbar as an application window tab bar?