It seems that sometimes in secular music, the emphasis is on the tune more than the lyrics. Some lyrics of course make no sence. ie.. "Louie Louie". So we naturally will hum the tune forever and not know but maybe a few of the lyrics. (ever see those tv shows?)
With Christian music, the emphasis is just the opposite. It's all about the lyrics. If the goal is to reach people where they are, why not take a great song and maybe add lyrics and now they hum the song and remember the lyrics?
On a side note, there are some songs that are so huge and classic unto themselves that we should leave alone.. ie.. The Thrill is gone, almost anything by the Eagles and Beatles.
Just my 2cents worth.