I don't think you can really compare a content creation application like Word with a content consumption application like a browser because people use them differently. Tabs might be useful in Word sometimes but there are other times where they would be a real pain. For instance, I often have multiple documents open side by side so I can easily cut and paste from one document to another using ALT+TAB to switch back and forth between documents. This would be difficult to do without using your mouse if all documents opened as tabs in a single instance of Word. Excel behaves in this way (without tabs - it acts as a sort of MDI interface) and it always annoys me.
Yes, like I said, I heard all of the same arguments for browsers back in the day. Heck it even took me a while to get used to tab web browsing. But now I can't imagine life without it. The take home message I think is if it's done properly, tabbed interfaces can work in a number of scenarios. I wouldn't want MS to take the approach that they did recently with Windows 8 and make it so that a somewhat different way of working was forced on those who didn't care for it. (I.e. the "do it my way, or the highway" approach to interface design.) Users should still have a choice, just as they do with tabbed web browsers, where in theory if you don't like tabs no one forces you to use them. However most people would agree that it would barely make any sense not to use tabs for this purpose now.
You are correct to some extent however. It depends on the usage scenario and on user preference. My frustration is born of the need to frequently have several tens of Word documents open, or PowerPoint slide shows, or indeed as someone else mentioned Explorer windows. My usage pattern under these conditions is often very browser like, with the need to frequently switch between different windows and documents.
The take home message is I think that if done right (and the 'done right' part should not be underplayed I think) that a tabbed user interlace could have many uses. Not so long ago people would have thought it was crazy to have a tabbed email client. However Thunderbird (although I rarely use it as my main client), appears to have implemented this rather well.
As another poster commented however, MS has seemed to 'lose the plot' a little with Windows 8, and have almost abandoned the paradigm of multitasking entirely, particularly with their new touch screen interface and lack of focus on providing a great desktop experience. It is not a helpful direction, particularly with those who may often be presented with the need to perform several complex tasks at the same time.
Just a closing note too. It might be worth thinking that added travel time to the taskbar may only equate to a couple of seconds at a time. But add those seconds up over a lifetime and they will probably equate to hours. Hours that you probably could spend doing something else far more enjoyable!