UiBinder does *not* depend on Composite. You can use it anywhere you can use a widget (assuming you are building Widgets in your ui.xml file).
returns a Widget - you can use this anywhere you would normally use a widget. In a Window, you could use setWidget, in a class that just implements IsWidget, you could keep a reference to it and return it from the asWidget() method. In fact, it isn't even required that you keep a reference at all - if you put a ui:field on the root widget and have a field in your java class for it.
Uses ui:field on root widget in ui.xml:
Keeps value of createAndBind and returns it