Whether you need the assets folder is up to your application. It is intended to hold things that are not dependent on a particular theme. You might have common scripts that should be available to all themes, like jquery. You might store user uploaded images, etc.
As for the cascading nature of assets, views and layouts... not every application will use it, or need it. For smaller apps it is overkill, definitely, and you probably won't want to use it. For larger apps it can be a blessing. So, the power and flexibility is there if you need it, but not required that you use it.