What do you do about the USB cable though? A flash drive you can plug in directly, it's guaranteed to work. A phone requires you to either carry around an extra

What do you do about the USB cable though? A flash drive you can plug in directly, it's guaranteed to work. A phone requires you to either carry around an extra cable (arguably more annoying to carry than a flash drive) or take the risk that you won't have the right cable available nearby when you're trying to (say) boot a laptop.