The advantages of natural language interfaces are that they are easy to learn since most people can speak their own language. They are easy to remember, although the ease of remembering depends on the semantics the application demands; the less natural the semantics the harder it will be to remember.
Transferring between applications using speech as an input should have less problems, since they should all use the same language as a basis. NL interfaces are powerful although this depends on the application's functional domain; the wider the function domain the greater the expressiveness of the application.
NL interfaces are flexible due to the many and varied phrases possible, they are fast and efficient for the user to utilise as they do not have to navigate through screens of forms or menus in order to reach the desired goal, rather they can combine the action into a single command.
NL interfaces demand moderate screen real estate, with the only real necessity being a small amount of screen estate for a feedback and clarification dialogue.