Metal is music. It's all music. What these people don't understand is that all music represents an emotion. Dance music is for people who like to dance. Pop is usually for people who want to feel good etc. Metal, is an expression of disgust with society... rage... rebellion and a feeling of "we're not taking this shit laying down anymore.. fuck it... lets FIGHT".
It's as valid as any other form of expression. It's just that people don't understand the expression of anger.