Is watching television useful? For what? Do you mean "Is watching television useful for learning a language?" If so, then I'd argue that there are more efficient ways to absorb a language, for example, speaking to native speakers and forcing yourself to respond in their language. On the other hand, watching TV if you are actively watching and trying to understand the speech is valid. I doubt that passively watching TV is very useful until you've reached an intermediate or advanced level in a language.