Genres are a way of compartmentalising music into categories in order to enable listeners to judge a particular artist or song without listening to a great deal more from them, because that's how human beings think. Just because "ridiculous" genres exist doesn't mean that it doesn't work, nonetheless, to help a user form an educated opinion of what they're listening to.

And there's no reason why a genre isn't valid - a genre is not defined by the songs in it, but rather a song is categorised into a genre. To say that songs can't be grouped according to similar features is like saying that classifying food into areas in a supermarket is invalid, since it should all just be considered "food." Obviously genres aren't a black-and-white thing - you'd have to be a fool to argue that - and there's always going to be some crossover between them, but to me there's a pretty clear distinction between the double-kicking and distorted guitars of a metal song and the thumping bass and atmospheric synths of a trance song.