Sure, when my iPhone is the only screen I have at that moment. Soon, when the governments ban laptops and tablets on flights, many more people will be watching shows on their phones.
To be honest, smartphones have the same capability as laptops and tablets. So if they ban those then I'd expect them to ban smartphones as well.
Size is the difference. I thought they are already banning laptops on flights from Middle East to USA, but not phones.
I never do and was wondering if others did. But then again, I don't watch movies much.