I suppose I should be more careful when writing it (after all, my nose is quite dear to me), but I find Leigh's comment about "a series of movies/books about wizards and wizardry that do not serve any purpose in learning English" very amusing. I too hardly ever talk about wizards and wizardry in everyday life, but this series is one of the main reasons why I can write this comment in English (maybe not absolutely correct and natural English, but still, it's better than nothing). Why stop at "Harry Potter", though? I suppose it makes sense to forget about William Shakespeare, Walter Scott, Jane Austen and other useless authors: their English is very outdated, God forbid people use their books in order to learn something about the English language. Because, you know these hopeless people, language learners: they are inherently unable to use critical thinking, they just parrot things they read and can't learn to differentiate between a literary device and "proper" English. 

Off-topic things aside, I think we should not completely depend on anything or anybody when learning a language, really. It's not just about songs, films and literature, it's the same with people: someone thrice older than me probably doesn't speak the way someone my age does; even people my age do not speak in the same way, it depends on where they're from, what level of education they have, what style they prefer etc. But I don't think learning from someone different from you is wasting time. Actually I'm pretty sure even learning things like Eminem's "Careful What You Wish For" by heart when I was 14 and didn't really speak English (for some reason lovely colour songs were not as interesting to me at the time) was beneficial for me in terms of language learning. 

I get your point, Richard. The only reason I decided to comment is that many learners read your posts and follow your advice (and rightly so) and I wouldn't want them to feel like they should avoid using songs and movies, because I think the pros outweigh the cons. As a non-native speaker of English, I can tell you that there's nothing quite like finally knowing the meaning of a song you've known for a long time or understanding every word in your favourite movie scene. These are the "wow" moments that make learning an amazing experience. Picking up a few non-standard expressions along the way seems like a minor problem to me. Besides, a simple Google search will tell you they're non-standard. Song lyrics are often discussed on various forums where native speakers patiently explain that this is not the way to speak to your boss :)

Oh and of course Pink Floyd's "leave them kids alone" (in my previous comment) is non-standard. Don't want to confuse learners.