In my opinion: none! People who sing in English rarely sing clearly. There are many songs where even native speakers can't understand what is being said. Also songwriters often mangle the language - they make up words, or use them inappropriately, to fit the rhyme or the tune. If you browse the questions here - you will find many questions asking the meaning of a phrase that may in fact have no meaning at all... I wondered why people thought that songs were a good way to learn - until I went to China, and realised that songs are actually sung clearly, and that I could understand the words better than I could understand people speaking.