What's the more common action: selecting a tab, or moving a window to a different screen?
It's not about which action is more common, its about the fact that the titlebar drag gesture is universal cross windows 7/8 and is a key element of the user experience. There are exactly two apps I've used in the entire history of windows 7 that broke this interaction: Chrome and FF. The fault is on those browsers breaking expected behavior, not Opera for doing what windows users expect. The FF UX team admitted that they overlooked this but its understandable that they're keeping it as users have come to expect it.
I'm really hesitant to say Firefox gets it "wrong" even if it is inconsistent.
Also calling chrome or Firefox window "just a holder of tabs" isn't fair, they are very special holders of very special tabs, and lots of work goes into them to make them work properly. Also there are other "holder of tabs" windows, e.g Sublime Text, which are mostly similar but have important differences. Trying to move all that complexity into WM simply won't work.