I can think of very few tasks where you "need" more than two or three windows open at any given time. My work rarely requires more than two. The rest can sit off the side of the screen, and wait for me to come back to them.
Now, unless you're some hotshot, "power user" still stuck in the past, pretending that a zillion and one things cluttered all over the screen, is "cool", and somehow makes you think you're better than everyone else, Metro will work for you, and does not prohibit any kind of productivity. It also adds functionality you'll never get on the desktop. It's just a different way of displaying and using what we've already learned from using Windows these past 20 years.
Um, there are jobs that require you to compare to things at once. How can you EASILY accomplish this? Oh yeah, to see them BOTH AT ONCE. Need recreate a PDF in InDesign almost perfectly? Can you imaging having to tab out thousands of times during your work day so you can compare the two to make sure they are very identical?
Most of these examples I am providing say two monitors. But of course with the way Non-metro windows works, you can resize things if you only have one monitor.
Doing a podcast also comes to mind. You can make sure everybody's audio is recording on one screen (or the audio recording software has not crashed), and have your research material (for discussion or whatever the podcast is about) on the other monitor. If you are using Skype and are waiting for a guest to arrive, you can have that window always visible to you can just glance at it every few seconds instead of tabbing to it.
Programming tutorials really benefit from having TWO THINGS open at once for a long period of time. This is what I do, I have the finished sample on one screen, and the screen that is recording on the other monitor.
Website browsing + productivity software really benefits from having them both open and visible at once.
Playing a borderless full screen game (which means tabbing out does not take a couple of seconds) while also talking to your friends through a separate IM client (non steam or xfire if that is still around) on another monitor.
Sometimes....it takes more time to tab between applications than it is to simply move your mouse and set its focus. Do I need to go on with these examples?