I don't agree with your argument because of songs I improve my english and I'm very good at this language now! If you want to know how that I do to understand english, I listened to english songs and search the French translator of this song, and I mixed up the good lyrics english/french ones together.. it wad my opinion ^^ But it can be true for some people that this type of learning doesn't work..

I agree in general. With some languages, songs are typically sung in a way that is clearer than normal speech, and they are therefore easy to understand, and it's a nice way to learn. But typically in English, the lyrics are indistinct, and the language is mangled to fit the tune. People regularly ask questions here about song lyrics that often don't make any sense at all, and at best we have to guess at the meaning. Many native speakers also have stories about how they thought that the singer was singing one thing, then they read the lyrics, and found it was something completely different. You can google 'misheard lyrics' for some funny examples.

Some notice improvement with listening skills and with grammar. But only some, that's why I agree with you.