I find it hard to find a case where tabs are not productive. Of course, you can do unproductive things in tabs as well as in windows, but in any event you'll do it faster in tabs.

For instance, my "workflow" for checking this board is that I have a link on my Safari Bookmarks Bar (as link #3) to the "Show new posts since last visit" function on the front page. So from within Safari i type Command-3, which opens up the list of new posts, then I scan the list and Command-click any interesting posts (either the whole post or just "New posts", depending on if I've read the post earlier), and once I have everything I want to read lined up in tabs, I click "Mark all topics as read", close the first tab and then read through each thread, closing tabs as I go. Similar to how many people use tabs for Google results etc.

Now, naturally when reading this board there's a fine line between productivity and dicking around, but the fact is that I would have read all the same threads anyway, but if I'd done it in seperate windows it just would've taken me more time and my desktop would've looked like a mess as I went along. Or if I'd open the links one at a time in the same window, it would've been annoying to have to go back all the time, and I might have forgotten along the way that I was actually going through the list of new posts.

Tabs are just more "Zen"... :)