I disagree.

The purpose is to save taskbar space. When I'm at work, I usually have one adobe photoshop, at least three windows explorers, one ssh, dreamweaver, and a couple of gVims. So when I like to browse, I will end up with a hundred buttons on the taskbar. I can either group similar programs, but that's more confusing, or I can use (as I do now) Phoenix's tabs to have all the sites I'm visiting in a single window that I can easily find in between the whole mess on the taskbar, and doesn't get confused with everything else.