I don't see how you can distinguish between a lyric and a formal prayer during worship services.
Can you express to your wife and/or children that you love them without resorting to a pre-written lyric ?
I must say that I have "a thing" about extempraneous prayers, since they often seem to be more directed AT other people than God.
I'm really speaking to the issue of private prayer (i.e. private interaction with God). Thus, ... no other people to impress.