You really ought not to be thinking, 'I must use metro apps'. The duality of Windows 8.x is the whole point. You have the choice.
Personally I don't think I'm ever going to launch that metro calculator on the desktop. I don't need nor want it taking up that much space. Also, I often use multiple calculator instances, so the metro thing isn't going to work for me anyway.
But on the tablet side, I'm sick of launching the desktop and opening a fiddly calculator. So the calculator app is a very welcome addition.
Even in 8.1 I still don't think metro apps blend very well with the desktop. There are some exceptions of course, like the twitter app or things that run in the background like audible or the music app. Or, if you have multiple monitors, having a metro app on one of them works well.
If you're on a desktop, stick as close as you can to your normal desktop workflow. You'll probably find that you're only interacting with the metro stuff about 10% of the time, or less. And give yourself time (months) to get used to it.
If you jump in head first you're probably going to come out hating it. But if you're pragmatic about getting used to the new UI you'll probably find it's not actually that different than using Windows 7.