I can see tasks where I might not need multi-windows like generic web-browsing (e.g. reading Hacker News). But in general I often have multiple things going on

I can see tasks where I might not need multi-windows like generic web-browsing (e.g. reading Hacker News). But in general I often have multiple things going on concurrently, be it video-chat + work, debugger + IDE, responding to an e-mail (having two e-mails open concurrently is useful), or just any task involving assembling information (e.g. searching with one window while building a "list" in a second). The Metro/tablet style of interface is useful for content consumption, but as soon as you move into the realm of content creation then it starts to get tricky.