That is the only reason I've ever pondered rooting my device: to apply the 'universal' Multi-Window hack. If all apps supported this I'd be happy and probably use it more.
The one case I use it the most is when taking notes and reading e-mails or browsing a web page. Chrome used to support multiple instances (separate windows for different pages), but not anymore, so when I want to compare or browse two pages, I have to use Chrome and the stock browser.
I would use Netflix and something else (probably Facebook), but Netflix is 'so smart' that even though you're in multiwindow, if that's not the active window, video stops. Good one, Netflix... ¬¬
Oh, and when I'm reading at night I usually keep Play Books in one half and an IM app to talk to friends/family on the other (it helps that the keyboard can be made to float to keep the screen from jumping up and down when the keyboard appears).